Support access control policy filtering on GRPC Proxies

[ravenac95]

We need to expose the access control policy for GRPC proxies to use to filter traffic ingress

Details

We would like to support an architecture that would look something like this for grpc and libp2p proxies:

General Ideas:

• The proxy layer should terminate TLS and validate the Client TLS certificate is from a valid node identity in the access control policy.

Acceptance Criteria

• An interface is designed to properly expose the required access control policy to support a GRPC proxy
• The Ekiden-Node can be configured to accept proxied requests over GRPC.