feat(extension/googleclientauth): support id-token

[rinx]

This PR adds support for generating id-token.

I'm planning to deploy OpenTelemetry collector as a Cloud Run application.
I'd like to use Cloud Run's service-to-service authentication or Cloud IAP's programmatic authentication to keep it private.

By supporting id-token in the googleclientauth extension, OpenTelemetry collector can be used as a client for the private collector deployed as a Cloud Run.

[dashpole]

I'm planning to deploy OpenTelemetry collector as a Cloud Run application.

To clarify, you are planning to run the OTel collector as a service in cloud run, and then use a different collector running elsewhere to send to the cloud run service?

[rinx]

To clarify, you are planning to run the OTel collector as a service in cloud run, and then use a different collector running elsewhere to send to the cloud run service?

That's right.
I'd like to run an OTel collector as a receiver service in Cloud Run, and deploy "sender" OTel collectors somewhere else (e.g. Cloud Run, GKE, or AWS services via workload identity federation).
This PR is for the "sender" role collectors.

[dashpole]

/gcbrun

[codecov]

Codecov Report

Attention: Patch coverage is 90.00000% with 2 lines in your changes are missing coverage. Please review.

Project coverage is 62.81%. Comparing base (4caace7) to head (e2e0330).
Report is 17 commits behind head on main.

Files	Patch %	Lines
extension/googleclientauthextension/factory.go	85.71%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #851      +/-   ##
==========================================
+ Coverage   61.03%   62.81%   +1.77%     
==========================================
  Files          56       57       +1     
  Lines        5903     4935     -968     
==========================================
- Hits         3603     3100     -503     
+ Misses       2143     1675     -468     
- Partials      157      160       +3     

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[dashpole]

can you update http_test and grpc_test with cases that use the id_token token type, and check the resulting headers?

[dashpole]

/gcbrun

[rinx]

can you update http_test and grpc_test with cases that use the id_token token type, and check the resulting headers?

added test cases that uses id_token.
However, I couldn't write a case for checking headers, because google.golang.org/api/idtoken requires a real GCP credentials.json for generating a token.

I've already confirm that this code works correctly in my real GCP project.

[dashpole]

/gcbrun

[rinx]

@damemi Could you please review this PR again?

[rinx]

@dashpole @damemi Thanks!