channelz: export local certificate used by a TLS connection

[easwars]

Our channelz implementation does not export local certificate used used by a connection. See here. https://github.com/grpc/grpc-go/blob/dc77d7ffe311f78f2e577572d984af3c0a8df82b/credentials/tls.go#L52-L55

As mentioned in the comment, this is because the tls package does not export this. There is an open proposal in the Go repo to make this happen. See: https://github.com/golang/go/issues/24673.

PSM Security interop tests also make use of this field in channelz to verify if a secure connection is using exactly the expected certificates. Currently, we have a way to skip this check in the interop test driver. But we would eventually need this support, both for our interop tests and for our users.

[easwars]

There is still no update on the issue in the Go repo.