Add helper method to get base64 encoded `Message`

[mahnunchik]

I've faced with the task of calculating the fee for a transaction.

Solana has RPC method getFeeForMessage https://docs.solana.com/ru/api/http#getfeeformessage

But the parameter is Message not a Transaction. Library doesn't allow to get Message directly and keys specific logic are incapsulated in the Transaction encoder.

Current solution to get a Message:

const tx = sol.createTx(address, toAddress, amount, fee, blockhash);
const decodedTx = sol.TransactionRaw.decode(base64.decode(tx));
const message = base64.encode(sol.Message.encode(decodedTx.msg)));

I think it is better to split logic between Message, Transaction, and TransactionRaw to be able to get specific data.