Move `substrate-bridge-relay` into repository (#1)

* Initial commit. CLI which parses RPC urls.

* Establish ws connections and make simple RPC requests.

* Complete bridge setup.

* Process subscription events.

* Ctrl-C handler.

* Write a bare-bones README and copy in design doc.

* Modularize code a little bit.

* Communicate with each chain in a separate task.

* Parse headers from RPC subscription notifications.

* Send (fake) extrinsics across bridge channels.

And now it's deadlocked.

* Fix deadlock.

* Clarify in README that this is not-in-progress.

* Move everything into a single folder

* Move Substrate relay into appropriate folder

* Get the Substrate Relay node compiling

* Update Cargo.lock

* Use new composite accounts from Substrate

* Remove specification document

It has been moved to the Wiki on the Github repo.

* Update author + remove comments

* Use latest master for jsonrpsee

Required renaming some stuff (e.g Client -> RawClient)

C...