Improve payload methods

[QYuQianchen]

This could be more idiomatic in Rust by:

1. creating a type encompassing the data necessary for the transformation:

pub struct TransferHoprTokenPayload {
    token_address: Address,
    address: Address,
    amount: U256,
}

2. creating a TryFrom implementation for the TransactionRequest for this:

impl TryFrom<TransferHoprTokenPayload> for TransactionRequest {
}

Objects from 1. could be exported for public use, and used, e.g. in edgli to easily create a hidden transformation inside the library code, e.g. implementing the hopli library calls in terms of convertability, e.g.:

pub fn execute_payload_against(v: TryInto<TransactionRequest>)...

Originally posted by @Teebor-Choka in #15 (comment)

[linear]

HOP-507