Implement enclave APIs that facilitate Fog View Router dynamic store discovery

[samdealy]

Motivation

The Fog View Router is responsible for taking incoming client requests, forwarding them to Fog View Shards, and then returning these received responses to the client. This PR adds three ake enclave methods (and corresponding view enclave methods) that facilitate this process.

The first method, backend_init, allows an enclave to create a Noise auth request to another enclave that serves as a backend.A backend connection is one in which the current enclave is a client to another enclave. It is not the same as a peer connection, in which the current enclave sends peer messages to another enclave, and it's distinct from a client connection, in which the current enclave services client requests. The backend_connect method finishes the Noise connection process, and the reencrypt_client_message_for_backends takes a client message and makes a message for each connected backend.

Addresses the `Implement shard component discovery in Fog Router" item in #2028. This item will be marked complete once the next PR, which uses these enclave methods to perform the dynamic store discovery, is merged.

This PR implements enclave methods described in the design doc's "Option 1: 'On The Fly' Authentication" section. The one slight difference is the shape of the Map and that it's stored in the ake enclave rather than the Fog View enclave. (It went from being Map<LoadBalancerHostPort, Map<StoreHostPort, Ready<_>> to Mutex<LruCache<ResponderId, Ready<Aes256Gcm>>>.

Future Work

• FogViewRouterService uses the reencrypt_client_message_for_backends method to create query requests for each Fog View Store
• FogViewRouterService uses the auth enclave methods to authenticate and connect to Fog View Stores via the "dynamic discovery" method.

[remoun]

Looking good!