[xDS] A65: mTLS Credentials in xDS Bootstrap File

[qudongfang]

Use case(s) - what problem will this feature solve?

Implement gRFC A65 so that users can configure mTLS for communicating with the xDS server via the xDS bootstrap file.

Proposed Solution

Alternatives Considered

Additional Context

• A65: mTLS Credentials in xDS Bootstrap File proposal#372
• [xDS] support "tls" channel creds in bootstrap file grpc#33234

[dfawley]

We have not planned to work on this during the current quarter, but it is on our radar and can probably be prioritized for next quarter. We're also able to do code reviews on PRs if you'd like to contribute this support. Thanks!

[dfawley]

Let's track with #6690 even though this one is older.