Add `auto_fill_txn`

[LimpidCrypto]

High Level Overview of Change

Adds transaction auto filling feature. Transactions have fields that can be filled automatically. This PR is required for #71

Context of Change

Type of Change

• [x] New feature (non-breaking change which adds functionality)

TODO

• [x] fix mutable borrow issues
• [x] add testing
• [x] handle unwraps