Chrome OS gRPC garcon Command Execution

There is a variety of RPC communication channels between the Chrome OS host system and the crosvm guest. This bug report focuses on communication on TCP port 8889, which is used by the "garcon" service. garcon uses gRPC, which is an RPC protocol that sends protobufs over plaintext HTTP/2. (Other system components communicate with the VM over gRPC-over-vsock, but garcon uses gRPC-over-TCP.) For some command types, the TCP connection is initiated by the host; for others, it is initiated by the guest. Both guest and host are listening on [::]:8889; however, the iptables rules of the host prevent an outside host from simply connecting to those sockets. However, apps running on the host are not affected by such restrictions.