…ic#246655)

## Summary

Closes elastic/search-team#12248

~WIP. Don't review yet.~

This branch implements ~a _rough_, proof-of-concept,~ Authorization Code
Grant Flow for the (yet unmerged) Sharepoint Online v2 connector, as
well as for the merged (modified here) Notion v2 connector.

The goal is to use this as a concrete example to frame discussion
around, and it iterate off of.


![code-grant-flow](https://github.com/user-attachments/assets/a9c06f4f-287b-42ef-b7da-653c89ac6375)

**Initial implementation:**
@seanstory: This has been entirely generated by code assistants, so
thorough review will eventually be needed, _especially_ because this is
an implementation of a hairy security protocol.
Also, this implementation is missing a LOT of necessary bits. Some of
which are:

1. cleanup of stale state with taskmanager task ✅
2. get refresh tokens working ✅
3. unit tests 🟡 
4. check on rate limiting of these endpoints? Potential resource
exhaustion ✅
5. prompt for `tenant-id`, instead of auth URL and token URL?
6. redirect to actual page, not HTML template ✅ 
    1. PM feedback first
7. telemetry
8. documentation 🟡 

**Additional changes:**
@lorenabalan:
elastic#246655 (comment)

### What's in the box?

1. New Auth Type (oauth_authorization_code.ts)
- Zod schema defining OAuth configuration (authorizationUrl, tokenUrl,
clientId, clientSecret, scope)
- configure() method that retrieves and refreshes tokens via
ctx.getToken()
  - Integration with existing connector auth framework

  2. OAuth State Management (oauth_state_client.ts)
  - PKCE state/verifier generation using crypto.randomBytes
  - State storage in new oauth_state saved object type
  - 10-minute expiration with cleanup capability
  - Encrypted storage of code_verifier

  3. Token Management (extended connector_token_client.ts)
- New methods: createWithRefreshToken(), updateWithRefreshToken(),
getRefreshToken()
  - Encrypted storage of access_token and refresh_token
  - Tracks expiration for both tokens

  4. HTTP Routes
- POST /api/actions/connector/{id}/_oauth_authorize - Initiates OAuth
flow, returns authorization URL
- GET /api/actions/connector/_oauth_callback - Handles OAuth callback,
exchanges code for tokens

  5. Token Retrieval (get_oauth_authorization_code_access_token.ts)
  - Fetches stored access token
  - Auto-refreshes expired tokens using refresh token
  - Returns null if authorization needed

  6. OAuth Token Requests
- request_oauth_authorization_code_token.ts - Exchanges authorization
code for tokens
  - request_oauth_refresh_token.ts - Refreshes access token

  7. Integration Points
- Updated ActionsClient.getOAuthAccessToken() to support
"authorization_code" type
- Modified getAxiosInstanceWithAuth() to call
getOAuthAuthorizationCodeAccessToken()
  - Added 401 interceptor with user-friendly error messages

  8. UI Components
  - useOAuthAuthorize hook - Calls authorize endpoint and opens popup
- Updated connector flyout footer with "Authorize" button for OAuth
connectors
  - checkOAuthAuthCode() helper to detect OAuth auth type

  9. SharePoint Online Connector
  - Changed from undefined auth to oauth_authorization_code
- Defaults for Microsoft OAuth endpoints (with {tenant-id} placeholder)
  - Scope: https://graph.microsoft.com/.default offline_access


### Checklist

Check the PR satisfies following conditions. 

Reviewers should verify this PR satisfies this list as well.

- [ ] Any text added follows [EUI's writing
guidelines](https://elastic.github.io/eui/#/guidelines/writing), uses
sentence case text and includes [i18n
support](https://github.com/elastic/kibana/blob/main/src/platform/packages/shared/kbn-i18n/README.md)
- [ ]
[Documentation](https://www.elastic.co/guide/en/kibana/master/development-documentation.html)
was added for features that require explanation or tutorials
- [ ] [Unit or functional
tests](https://www.elastic.co/guide/en/kibana/master/development-tests.html)
were updated or added to match the most common scenarios
- [ ] If a plugin configuration key changed, check if it needs to be
allowlisted in the cloud and added to the [docker
list](https://github.com/elastic/kibana/blob/main/src/dev/build/tasks/os_packages/docker_generator/resources/base/bin/kibana-docker)
- [ ] This was checked for breaking HTTP API changes, and any breaking
changes have been approved by the breaking-change committee. The
`release_note:breaking` label should be applied in these situations.
- [ ] [Flaky Test
Runner](https://ci-stats.kibana.dev/trigger_flaky_test_runner/1) was
used on any tests changed
- [ ] The PR description includes the appropriate Release Notes section,
and the correct `release_note:*` label is applied per the
[guidelines](https://www.elastic.co/guide/en/kibana/master/contributing.html#kibana-release-notes-process)
- [ ] Review the [backport
guidelines](https://docs.google.com/document/d/1VyN5k91e5OVumlc0Gb9RPa3h1ewuPE705nRtioPiTvY/edit?usp=sharing)
and apply applicable `backport:*` labels.

### Identify risks

Does this PR introduce any risks? For example, consider risks like hard
to test bugs, performance regression, potential of data loss.

Describe the risk, its severity, and mitigation for each identified
risk. Invite stakeholders and evaluate how to proceed before merging.

- [ ] [See some risk
examples](https://github.com/elastic/kibana/blob/main/RISK_MATRIX.mdx)
- [ ] ...

---------

Co-authored-by: kibanamachine <42973632+kibanamachine@users.noreply.github.com>
Co-authored-by: Lorena Bălan <lorena.balan@elastic.co>

…n + cleaned up dead code + Edited search_messages.yaml to not provide display name guidance that is hyper-specific to Elastic