tezos-proxy-server should forward unknown requests to the node

Context

The current implementation of tezos-proxy-server supports a subset of RPC requests. Instead, can we forward any unknown request to the node? Worst case, the node will return 4xx.

This way, no one can tell the difference between interacting with the node and interacting with the proxy. An operator can safely set up a proxy in front of a node regardless of the use case.

This would make for a great improvement of the usability of tezos-proxy-server.

Investigation

Section moved in comments, for better seeing the history

Summary: we will introduce a way to customize return codes in resto, as detailed in this thread. Then we will use this feature to return Moved_permanently on requests not served by tezos-proxy-server, specifying to use the node's address instead.