Add an experimental googleclientauth extension

[dashpole]

Similar to https://github.com/open-telemetry/opentelemetry-collector-contrib/blob/main/extension/oauth2clientauthextension, but supports google application default credentials.

This extension:

• Gets application default credentials
• Uses project and quota project, if those are set in the credentials.
• Ensures the user has set a project

Things I didn't do, but could have:

• Add impersonation, similar to the googlecloud collector exporter
• Support X-Goog-Request-Reason, or other system parameters.
• Support other CredentialsParams that the client library supports when getting ADC.

Testing: I don't have a way to test this right now, but it seems simple enough that it should work.

[codecov]

Codecov Report

Merging #703 (e55d34f) into main (fff463f) will increase coverage by 0.07%.
Report is 6 commits behind head on main.
The diff coverage is 69.76%.

@@            Coverage Diff             @@
##             main     #703      +/-   ##
==========================================
+ Coverage   69.59%   69.66%   +0.07%     
==========================================
  Files          36       42       +6     
  Lines        4717     4840     +123     
==========================================
+ Hits         3283     3372      +89     
- Misses       1286     1317      +31     
- Partials      148      151       +3     

Files Changed	Coverage Δ
extension/googleclientauthextension/grpc.go	0.00% <0.00%> (ø)
extension/googleclientauthextension/factory.go	71.42% <71.42%> (ø)
extension/googleclientauthextension/config.go	100.00% <100.00%> (ø)
extension/googleclientauthextension/http.go	100.00% <100.00%> (ø)

... and 13 files with indirect coverage changes

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[dashpole]

I think I've thought of a way to test this. We can support custom authenticators on our exporter in client config, and run integration tests using this as a source of auth

[dashpole]

I've decided not to do the integration test in this PR, so i've reverted that commit. I can do it as a follow-up, if it is needed.

[dashpole]

Through the experiment with testing, I have confirmed that the extension can be used with the cloud logging, monitoring, and trace APIs.

[aabmass]

LGTM

[aabmass]

Should the X be lowercase as well https://cloud.google.com/apis/docs/system-parameters#:~:text=HTTP%20request%20headers%20with%20keys%20in%20lowercase ?

[dashpole]

Based on what the current client does, the X is still capitalized: https://github.com/googleapis/google-api-go-client/blob/113082d14d54f188d1b6c34c652e416592fc51b5/transport/grpc/dial.go#L224