TLSInfo not available from ClientHandshake

[donovanhide]

https://github.com/grpc/grpc-go/blob/master/credentials/credentials.go#L168-L170

It is useful to receive access to the ConnectionState of a TLS connection when both server and client want to prove their identity in a peer to peer scenario at the application level. For example both peers might want to sign the TLSUnique bytes with their private key. This omission prevents that from happening.

[yacovm]

I agree, this is useful to defend against attacks such as described in:
https://mitls.org/pages/attacks/3SHAKE

[menghanl]

@MakMukhi is working on this. Will have a PR shotly.

[donovanhide]

Cool, thanks!