JSON-RPC client generation (#2778)

* Bump jsonrpc & generate clients.

* Initial version of JSON-RPC client.

* Re-wort

* Remove spurious `#[derive(Encode, Decode)]`

They did not compile, since `Encode` and `Decode` are deliberately not
implemented for `usize`.

Fixes #2742.

* Re-write rpc-client example.

* Update to jsonrpc=12.0.0

* Remove unnecessary import

* Bump version.

* Revert version bump.

* Bump again.