Introduce invite RPC requests

[mycognosist]

Another simple addition. Here we add the ability to send invite.create and invite.use RPC requests.

invite.create has an optional argument to specify the number of uses for the invite being created, ie. "generate an invite code that can be used x times" (where x is the value of uses).

[mycognosist]

I've changed the status on this to Draft while I track down a bug in the uses argument for invite.create.

[mycognosist]

The invite.create RPC method expects the argument to be in the shape of an object. The object must not be inside of an array. This is different to the other methods currently supported by kuska.

I have introduced the ArgType enum to allow us to match inside of send_request() and format the args field for BodyRef correctly:

pub enum ArgType {
    Array,
    Tuple,
    Object,
}

// inside of `send_request()` ...
// note the different representation of `args` in each case
let body_str = match arg_type {
    ArgType::Array => serde_json::to_string(&BodyRef {
        name,
        rpc_type,
        args: &[args],
    })?,
    ArgType::Tuple => serde_json::to_string(&BodyRefWithOpts {
        name,
        rpc_type,
        args: (args, opts),
    })?,
    ArgType::Object => serde_json::to_string(&BodyRef {
        name,
        rpc_type,
        args: &args,
    })?,
};

I can confirm this is working against the go-sbot.

CC: @adria0, this is now ready for review :)

[adria0]

LGTM with comments

[adria0]

I would prefer ArgsOptsTuple I think that it's more descriptive.