Add transaction worker

[banool]

Summary

This PR adds support for a transaction worker that can be used to submit transactions with a queue using a single account. It is possible to override how the transaction is submitted so you can use this with gas stations. The default behavior is to use direct submission.

There are a few things we should do in the future, I will turn them into issues:

• Support more than just ed25519 private keys as accounts.
• Use the gas estimation API (this should be encapsulated in the client, not in the txn worker).

I will turn these into issues.

Test Plan

aptos node run-localnet --force-restart --assume-yes

cargo test -p aptos-txn-worker -- test_worker --nocapture

cargo test -p aptos-txn-worker --doc