Breaking JSON RPC 2.0 changes

• When registering server.on methods you should no longer define the glz::expected return type. This is now hidden from the user and you can simply return your result type. If you want to return an error, simply return a rpc::error like rpc::error{rpc::error_e::server_error_lower, "my error"};
• See the jsonrpc documentation or look at the tests for more details
• Though this is a breaking change, it is a quality of life improvements and reduces code

Fixes

• Fixed reading empty struct with reflection (thanks @huangminghuang)