docs(samples): add auth samples and tests

[Sita04]

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[snippet-bot]

Here is the summary of changes.

You are about to add 5 region tags.

• samples/cloud-client/snippets/authenticate_explicit_with_adc.py:15, tag auth_cloud_explicit_adc
• samples/cloud-client/snippets/authenticate_implicit_with_adc.py:15, tag auth_cloud_implicit_adc
• samples/cloud-client/snippets/idtoken_from_metadata_server.py:15, tag auth_cloud_idtoken_metadata_server
• samples/cloud-client/snippets/idtoken_from_service_account.py:15, tag auth_cloud_idtoken_service_account
• samples/cloud-client/snippets/verify_google_idtoken.py:15, tag auth_cloud_verify_google_idtoken

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[piaxc]

Looks good Sita, thanks for the quick turnaround on these!

[Sita04]

Thank you @sai-sunder-s and @parthea for the commits.
Merging this PR.