Add test to verify encoding/decoding each field works

[jasonpaulos]

Summary

Recently this SDK had a bug where the transaction extra pages field, apep, was being ignored when decoding a transaction. In order to prevent bugs like this from happening again, we can create a test that automatically verifies each field persists during a roundtrip of encoding + decoding.

Scope

In order to do this, we'll need a list of each encoded transaction field key name. This must be updated with every new txn field.

Then, there are two options:

1. Make an object with every single field from that list present, then run it through a roundtrip of Transaction.get_obj_for_encoding and Transaction. from_obj_for_encoding and verify all fields are still present.
2. For each field from the list, make an object with ONLY that field present, then run it through a roundtrip of Transaction.get_obj_for_encoding and Transaction. from_obj_for_encoding and verify that field is still present.