Goal: accept 'transactions.Transaction' in addition to 'transactions.SignedTxn' when reading files

## Summary

The current goal code assumes that files it reads will be in the `transactions.SignedTxn` format. In many cases it is easier for SDKs to write the `transactions.Transaction` format. To increase compatibility goal should have a utility to convert `transactions.Transaction` to `transactions.SignedTxn` when possible.

## Scope/Requirements

1. Write a initializer function which accepts either format.
2. Look for all the goal commands which work with files and initialize the file using the new function.

## Urgency/Relative Priority