[electrs] `server.features` only available with the `electrum-discovery` feature

The [`server.features`](https://electrumx-spesmilo.readthedocs.io/en/latest/protocol-methods.html#server-features) call is one of the most basic Electrum calls that is for example a prerequisite to figure out which chain hash and protocol version a server speaks. However, `electrs` opts to only make this API call available when the `electrum-discovery` feature is set.

However, retrieving server features should be available even if the node won't be publically announced via the P2P network, as otherwise client can't rely on protocol version information or chain hash information being available.