Enable xDS credentials

[PapaCharlie]

This change should be relatively straightforward. It is a noop outside of the context of xDS (as demonstrated by the fact that the tests all pass), but it enables xDS-provided certificates (i.e. the ones that would be provided/specified in GRPC_XDS_BOOTSTRAP). See proposal A29 for additional detail.

[dragonsinth]

This probably needs a rebase. Are you sure that the XDS creds should wrap the errSignalingCreds and not the other way around? The code flow is a little tricky to absorb.

[PapaCharlie]

Let me check

[PapaCharlie]

I think you're right, it's likely best to wrap the final creds with the errSignalingCreds.

[PapaCharlie]

Yeah I confirmed that this works! Since errSignalingCreds embeds the credentials.TransportCredentials, it still implements the UsesXDS interface that's required by the code.

[dragonsinth]

Yeah I confirmed that this works! Since errSignalingCreds embeds the credentials.TransportCredentials, it still implements the UsesXDS interface that's required by the code.

Waaaait a second. That's definitely not true.

[dragonsinth]

I had to open another PR to get CI to work, for some reason it's getting stuck on this one.
#441

WDYT?

[PapaCharlie]

Yeah I confirmed that this works! Since errSignalingCreds embeds the credentials.TransportCredentials, it still implements the UsesXDS interface that's required by the code.

Waaaait a second. That's definitely not true.

Why not? This prints true: https://go.dev/play/p/TxQSrKPngtc?v=

[dragonsinth]

Why not? This prints true: https://go.dev/play/p/TxQSrKPngtc?v=

That one works because you embedded the concrete fooBar, which implements both interfaces. If you change it to this, it doesn't work:

https://go.dev/play/p/OyIwdHFyJ3x

(I had to rename the interfaces so that the interface names and method names dont clash.)

What we're trying to do here is more similar to this example.