Description: this is a follow up to #498. This PR introduces `envoy_engine_callbacks`. They are similar in nature to envoy_http_callbacks. The difference being that they are not exposed all the way to consumer level in the library as it is not needed right now. However, one can see how by adding a type erased context pointer, and following the platform patterns for http callbacks we could thread this all the way up if need be. The immediate need for these callbacks is to detach the engine's native thread from the JVM on Android.
Risk Level: med -- adds complexity to engine management.
Testing: local testing on devices (Lyft and example app on iOS and Android).

In conjunction with #498 this PR Fixes #492 #445

Signed-off-by: Jose Nino <jnino@lyft.com>
Signed-off-by: JP Simard <jp@jpsim.com>

Description: this is a follow up to #498. This PR introduces `envoy_engine_callbacks`. They are similar in nature to envoy_http_callbacks. The difference being that they are not exposed all the way to consumer level in the library as it is not needed right now. However, one can see how by adding a type erased context pointer, and following the platform patterns for http callbacks we could thread this all the way up if need be. The immediate need for these callbacks is to detach the engine's native thread from the JVM on Android.
Risk Level: med -- adds complexity to engine management.
Testing: local testing on devices (Lyft and example app on iOS and Android).

In conjunction with #498 this PR Fixes #492 #445

Signed-off-by: Jose Nino <jnino@lyft.com>
Signed-off-by: JP Simard <jp@jpsim.com>

**Commit Message**

- Implement Azure OpenAI integration including a) backend auth handler
b) Azure token rotator.
- Replace oauth package with `token_provider` to simplify AWS and Azure
token request logic.
- Fix existing test case i.e. in backendsecurity_test.go when oidc is
nil for other provider.
- Centralize some of string literals in `constants.go`

**Related Issues/PRs (if applicable)**
Related Issue: #347 
Related PR: #424

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Signed-off-by: Xiaolin Lin <xlin158@bloomberg.net>
Signed-off-by: Dan Sun <dsun20@bloomberg.net>
Co-authored-by: Dan Sun <dsun20@bloomberg.net>

**Commit Message**

Add proxy URL option for Azure authentication.


**Related Issues/PRs (if applicable)**
Related PR: #445

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Signed-off-by: Xiaolin Lin <xlin158@bloomberg.net>

**Commit Message**

Fix APISchema to support response_format.

**Related Issues/PRs (if applicable)**

Related PR: #445

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Signed-off-by: Xiaolin Lin <xlin158@bloomberg.net>

**Commit Message**

Updating support for accessing azure via OIDC in addition to the already
implemented access via access secret. Token retrieved from OIDC will be
exchanged for an azure access token which will be store the same way as
getting token via client secret/id and tenant id.

Key changes
- renamed azure_token_provider -> azure_secret_token_provider (since
it's client secret specific)
- generalized new azure_token_provider to have TokenProviders provider
federated tokens
- Azure tries to create token provider from oidc if available else
client secret

**Related Issues/PRs (if applicable)**

#445 

Tested on own cluster and verified that secret access token was
generated + same credentials appeared on extproc filter config file +
able to query azure

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Signed-off-by: Aaron Choo <achoo30@bloomberg.net>
Co-authored-by: Dan Sun <dsun20@bloomberg.net>