api: use generic types parameters

[broucz]

Use generic types parameters for Request and Response types in replacement of the Method, Params and Result types.

Example:

// From

pub enum Request {
    Batch(Vec<Payload>),
    Single(Payload),
}

// To

pub enum Request<Message, Params> {
    Batch(Vec<Payload<Message, Params>>),
    Single(Payload<Message, Params>),
}