Implement `re_grpc_server`

[jprochazk]

We need an in-memory pseudo storage node which only stores a single physical recording and supports the following calls:

service InMemoryStorageNode {
    rpc WriteMessages(stream LogMsg) returns ()
    rpc ReadMessages() returns (stream LogMsg)
}

Meaning it functions identically to re_sdk_comms and re_ws_comms. Eventually re_grpc_server should be expanded to work as a fully-featured in-memory storage node.

We should not switch to it yet, because we want to be more careful about breaking changes.