[Solana]: Support specifying fee payer for an encoded transaction

[10gic]

Is your feature request related to a problem? Please describe.
A fee payer on Solana is an account that covers the transaction fees for a user. The fee payer is the first writable signer account in the list of accounts for a transaction.

Fee payers can simplify the user experience by allowing users to interact with an app without needing to acquire the native token. Developers can sponsor these fees to make it easier for users to engage with their app.

It would be amazing if this library supported specifying a fee payer for an encoded transaction.

Describe the solution you'd like
Add a function likes: tw_solana_transaction_add_fee_payer(encoded_tx, fee_payer)

Describe alternatives you've considered

Checklist

Resources

https://solana.com/docs/core/fees

Additional context

[satoshiotomakan]

Thanks for opening the issue!
Regarding adding an external fee payer for an encoded transaction via tw_solana_transaction_add_fee_payer(encoded_tx, fee_payer) - I'm not against that, but in TrustWallet we don't plan to work on that right now.
Please feel free to open a PR