feat: add trip-to-trip and route-to-route fields to transfers

[KClough]

Summary:

Summarize the changes in the pull request including how it relates to any issues (include the #number, or link them).

#1114

Expected behavior:

Explain and/or show screenshots for how you expect the pull request to work in your testing (in case other devices exhibit different behavior).

• the ForeignKeyViolationNotice is triggered if these fields are referencing a value that doesn't exist.
• the UnknownColumnNotice is not triggered by these four fields.

Please make sure these boxes are checked before submitting your pull request - thanks!

• Run the unit tests with gradle test to make sure you didn't break anything
• Format the title like "feat: [new feature short description]". Title must follow the Conventional Commit Specification(https://www.conventionalcommits.org/en/v1.0.0/).
• Linked all relevant issues
• Include screenshot(s) showing how this pull request works and fixes the issue(s)

[maximearmstrong]

I modified the London Transit Commission dataset, which already implements transfers.txt, to test these changes.

Pre changes:

Post changes:

I confirm that the behavior is correct.

[maximearmstrong]

LGTM! Thanks @KClough