add modeling for related transactions

[varunpcb]

Fixes # .

Motivation

Adds support for the planned related transaction feature that will allow linking of multiple transactions within rosetta.

Solution

- Transactions will now feature an optional related_transactions array of RelatedTransaction objects. Related Transaction objects feature a network_identifier (optional), transaction_identifier, and direction (forward or backward). This was done to provide support for multiple relations of different directions across multiple networks if necessary. ### Open questions

[patrick-ogrady]

Make sure to populate the PR description...you'll also need to sign your commits before this can be merged

[varunpcb]

going to create a new pr to fix signatures.