feat(auth): add auth samples

[FrodoTheTrue]

[snippet-bot[bot]]

Here is the summary of changes.

You are about to add 6 region tags.

• auth/authenticate_explicit_with_adc.go:17, tag auth_cloud_explicit_adc
• auth/authenticate_implicit_with_adc.go:17, tag auth_cloud_implicit_adc
• auth/id_token_from_impersonated_credentials.go:17, tag auth_cloud_idtoken_impersonated_credentials
• auth/id_token_from_metadata_server.go:17, tag auth_cloud_idtoken_metadata_server
• auth/id_token_from_service_account.go:17, tag auth_cloud_idtoken_service_account
• auth/verify_google_id_token.go:17, tag auth_cloud_verify_google_idtoken

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[FrodoTheTrue]

@quartzmo could you please take a loot once more? Also, I am receiving stange lint error ...

To fix this check, run "goimports -w . && find . -name go.mod -execdir go mod tidy \;"
HEAD detached at pull/2706/merge
Changes not staged for commit:
  (use "git add <file>..." to update what will be committed)
  (use "git restore <file>..." to discard changes in working directory)
	modified:   auth/auth_test.go
	modified:   auth/id_token_from_impersonated_credentials.go

no changes added to commit (use "git add" and/or "git commit -a")
Error: Process completed with exit code 1.

Not sure what to do with that

[FrodoTheTrue]

@codyoss could you please help me with an error above?

[codyoss]

Looks like you need to run goimports to format the files.

[FrodoTheTrue]

oh, it's updated finally, thanks! waiting for the rest of the review for now :)

[FrodoTheTrue]

@quartzmo could you please take a look once more?

[FrodoTheTrue]

@codyoss could you please approve this?