JSON-RPC: add `rpc.discover` method

[link2xt]

This method should return OpenRPC schema: https://spec.open-rpc.org/#service-discovery-method

This way openrpc.json does not need to be distributed separately, but can be extracted from deltachat-rpc-server.

[r10s]

closing stale issue, this seems to be sufficiently solved by #4587, also all platforms use jsonrpc in the one or the other way already. if additional things are needed, they can go to a PR accompanying the concrete implementation