[extension/googleclientauth] Mark GCP client auth extension alpha

[damemi]

Description:
Follow up to #32029 and #31518
This marks the googleclientauthextension as alpha following the contributing doc and alpha stability guidelines. It also adds it to otelcontribcol

Note this also updates CONTRIBUTING.md to explain how to add the alpha component to otelcontribcol.

Link to tracking Issue: N/A

Testing: Downstream unit and integration tests

Documentation: No changes except stability

[damemi]

I must have done something wrong, because I'm stuck in a loop getting CI to pass.

I need to run make genotelcontribcol, which removes the replace in go.mod for the extension. Then I run make crosslink which removes go.mod references to our downstream code for the extension. Then, if I run go mod tidy in cmd/otelcontribcol, it gets me back to CI saying I need to run make genotelcontribcol again.

Is there a certain order I should run these?

[damemi]

Think I got it, I was missing the replace in builder-config.yaml, weird that crosslink will add that automatically to go.mod but you still need to add it manually to the builder config

[damemi]

@dashpole think I got it all sorted out, this is ready to merge

[damemi]

@djaglowski ping, this is ready for review/merge