Summary

Closes [WorkplaceAI] Define one Kibana stack connector from spec #12056

We want to add Notion as a "data connector", i.e. a source of data a user may want to connect and chat with. For this, we need a stack connector for the interaction with the 3rd party.

This PR is converting the original Notion ksc from POC into ksc "v2".

Related PRs ➡️ #245132 also enables us to send headers with every request, which is what we need for Notion (to specify the version).

Note for future self:

schema is for setting up custom connector-specific configuration. At this time I haven't identified a need for this for the Notion connector.
example of how to enable it in the future:

  schema: z.object({
    my_field: z.string(),
    method: z
      .enum(['GET', 'POST', 'PUT', 'PATCH', 'DELETE'])
      .meta({ label: 'Method' })
      .default('POST'),
    url: z.url().meta({ label: 'URL', placeholder: 'https://...' }),
  }),

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, uses sentence case text and includes i18n support
• Documentation was added for features that require explanation or tutorials
• Unit or functional tests 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
• 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 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
• Review the backport guidelines and apply applicable backport:* labels.