Add address labelling support to RPC API

[kristapsk]

I agree with the rest apart from the above two comments.

However, as with all such UI type changes, it makes sense to include an adaptation to both Qt and, especially, to the RPC interface, to allow clients to use them.

Originally posted by @AdamISZ in #1468 (comment)

There is currently none. Could be optional parameters when requesting new address and when creating transactions.

[AdamISZ]

Agree. Also adding 'help wanted' label since this would be a good way for someone to familiarize themselves with how to update the API. Essentially, you can see in e.g. #1461 that the changes needed are in docs/api/wallet_rpc.yaml (specification - this also auto-generates docs) and jmclient/jmclient/wallet_rpc.py (add method here .. other source files may or may not need changes, of course).