xDS: extension points for GoogleGrpc credentials

[markdroth]

Commit Message: xDS: extension points for GoogleGrpc credentials
Additional Description: Adds extension points for GoogleGrpc channel and call credentials.
Risk Level: None
Testing: N/A
Docs Changes: Included in PR
Release Notes: N/A
Platform Specific Features: N/A

[repokitteh-read-only]

As a reminder, PRs marked as draft will not be automatically assigned reviewers,
or be handled by maintainer-oncall triage.

Please mark your PR as ready when you want it to be reviewed!

🐱

Caused by: #40823 was opened by markdroth.

see: more, trace.

[repokitteh-read-only]

CC @envoyproxy/api-shepherds: Your approval is needed for changes made to (api/envoy/|docs/root/api-docs/).
envoyproxy/api-shepherds assignee is @mattklein123
CC @envoyproxy/api-watchers: FYI only for changes made to (api/envoy/|docs/root/api-docs/).

🐱

Caused by: #40823 was opened by markdroth.

see: more, trace.