Description:

The proposal is to upstream the ALTS support based on gRPC TSI (transport socket interface) implementation to Envoy main repo.

The current implementation have two part:

• gRPC TSI based TransportSocket implementation which may be reused to implement other gRPC based transport security (e.g. TLS)
• ALTS specific TransportSocketConfigFactory to instantiate the TransportSocket.

cc @alyssawilk @htuch

[optional Relevant Links:]
Existing implementation:
https://github.com/istio/proxy/tree/master/src/envoy/alts