Ops 2239/refactor get response schema

[rajohnson90]

What changed

Updated the GET Agreement and GET Agreements endpoints to use base marshmallow schemas in order to serialize Agreements before being returned by the API. We could further use marshmallow capabilities to exclude unneeded data and keep the Agreement object size smaller, but I don't have the knowledge of our codebase required to determine what data is currently considered 'unnecessary'.

Issue

Issue 2239

How to test

1. Open any agreement in the OPS dashboard
2. Verify that none of the fields are populated with placeholder values (such as TBD)

Definition of Done Checklist

• OESA: Code refactored for clarity
• OESA: Dependency rules followed
• Automated unit tests updated and passed
• Automated integration tests updated and passed
• Automated quality tests updated and passed
• Automated load tests updated and passed
• Automated a11y tests updated and passed
• Automated security tests updated and passed
• 90%+ Code coverage achieved
• Form validations updated

[rajohnson90]

Realized I did not map CANs in. I will need a bit more time to add CANs so I'm moving this back to draft status.