transport socket extension: gRPC TSI based ALTS support

[lizan]

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

[alyssawilk]

I believe this falls neatly under our new extension policy, so I think this should be good to go.

[stale]

This issue has been automatically marked as stale because it has not had activity in the last 30 days. It will be closed in the next 7 days unless it is tagged "help wanted" or other activity occurs. Thank you for your contributions.

[stale]

This issue has been automatically closed because it has not had activity in the last 37 days. If this issue is still valid, please ping a maintainer and ask them to label it as "help wanted". Thank you for your contributions.

[danzh2010]

I think this is not done yet.