Closes #2780

What this PR does

• Creates a new model EligibilityApiConfig for agency-specific Eligibility API data (public/private client keys)
• Data migration for existing TransitAgency with Eligibility API config
• Removes config fields from TransitAgency, adds optional foreign key to EligibilityApiConfig
• Creates a new model EligibilityApiVerificationRequest for flow-specific Eligibility API data (keys, JWT config)
• Data migration for existing EnrollmentFlow with Eligibility API config
• Remove config fields from EnrollmentFlow, adds optional foreign key to EligibilityApiVerificationRequest

The idea is to make configuration of flows simpler going forward, as we don't expect to use this API for new flows, simply maintaining the existing 2 in production (MST Courtesy Cards, SBMTD Reduced Fare Mobility ID).

The EligibilityApiConfig changes weren't necessarily spelled out in #2780. However they are similar enough, and similarly meant to make the TransitAgency configuration experience simpler for agencies going forward.

Testing the PR

User flow

1. Checkout the branch
2. Reset your database with local_fixtures.json from the branch
3. Launch the app with F5
4. Run through CST Agency Card eligibility
5. Expect to pass, see no errors

Admin (superuser)

1. Login to the admin as a superuser
2. Confirm you can see and change both new model classes and the corresponding foreign key fields

Admin (Cal-ITP user)

1. Login to the admin as calitp-user
2. Confirm you cannot see the model admins for either new class
3. Confirm you can see and change the corresponding foreign key fields