RPCs for reserved peers (#4423)

* RPC forwarders for adding reserved peers

* implement service side of reserved-peer RPCs

* Clean up unnecessary format! invocation

Co-Authored-By: Niklas Adolfsson <[email protected]>

* add some tests for the new RPC methods

* remove redundant `data` field

Co-Authored-By: Tomasz Drwięga <[email protected]>