A new transport socket extension for offloading mTLS handshakes to Secure Session Agent (S2A)

This issue introduces a new transport socket extension. This extension will offload the functionality of performing mTLS handshakes to the [Secure Session Agent](https://github.com/google/s2a-core/blob/main/ABOUT.md). The Secure Session Agent is a service that enables workloads to offload mTLS handshakes and protects the private key material from exfiltration.

Using this transport socket extension, the Envoy client will create a gRPC connection to the S2A handshaker service. The Envoy client will then relay the handshake messages between the S2A handshaker service and the peer. Once the mTLS handshake is complete, the Envoy client will receive the session key for encrypting/decrypting the record protocol messages.

This extension will provide a similar functionality as the ATLS transport socket extension but for offloading mTLS handshakes.