Make relay CLI generic (#849)

* Start generalizing rialto-millau commands.

* cargo fmt --all

* Introduce generic balance.

* Unify message payloads.

* cargo fmt --all

* init - generic

* Attempt to unify send message.

* Start moving things around.

* cargo fmt --all

* Move init-bridge.

* cargo fmt --all

* Improve UX of bridge argument.

* Fix clippy.

* Fix docs and scripts.

* Add docs.

* Apply suggestions from code review

Co-authored-by: Hernando Castano <[email protected]>

* Fix copyright.

* Add issue numbers.

* More todos.

* Update comments.

Co-authored-by: Hernando Castano <[email protected]>