feat: Request::into_multicall_struct()
feat: Request::into_multicall_struct()

This function serializes a `Request` into an XML-RPC struct that can be
used with the `system.multicall` method [0]. Specifically it creates a
struct with two fields: `methodName` for the method's name as a string
and `params` for the request's arguments as an array.

[0]: https://mirrors.talideon.com/articles/multicall.html

jonas-schievink/xml-rpc-rs