credentials/oauth: Distinguish between Unavailable and Unauthenticated errors

[dfawley]

See grpc/grpc-java#3267 (comment) for more information.

The fix is here:

grpc-go/transport/http2_client.go

Lines 378 to 381 in a62701e

	data, err := c.GetRequestMetadata(ctx, audience)
	if err != nil {
	return nil, streamErrorf(codes.Internal, "transport: %v", err)
	}

This should respect any status codes returned by the PerRPCCredentials call, and use UNAUTHENTICATED if one is not returned (instead of INTERNAL). The oauth implementation should return UNAVAILABLE if the oauth server cannot be reached.

[dfawley]

This has been blocked for 3 years, so I'm just going to close it. If anything happens on golang/oauth2#264, we can reopen this or file a new copy.