Allows a custom websocket address to be specified

[rmontagnin]

Feature description

Currently the websocket endpoint is computed as <rpc.address>/websocket, however this might be reductive for some cases where people want to specify a custom websocket endpoint.

Implementation proposal

We can add a new field to the rpc section:

[rpc]
addres = ""
websocket = ""

If websocket is specified, use that one. Otherwise, use rpc.address/websocket as the websocket endpoint.