cli: make rpc_interface re-useable

[niklasad1]

Follow-up on #13384

It changed the rpc_port from a Option<u16> -> u16 which broke overriding it in cumulus (see paritytech/cumulus#2529) as the default value was used instead

So let's expose rpc_interface so others can override the default port besides using the CLI --rpc-port

We could also revert port to be an option again but I would prefer to keep it port as u16 because it's simplifies the code in substrate but makes it a bit annoying to override (by comparing the port to the default sc_cli::RPC_DEFAULT_PORT)

polkadot companion: paritytech/polkadot#7190
cumulus companion: paritytech/cumulus#2538

[niklasad1]

let's do it with an option instead I don't like this anyway, I realized it will make the structopt docs weird