Research Form Pre-Fill Functionality in vets-api for Forms 21-22 & 21-22a

[holdenhinkle]

Issue Description:

The goal of this ticket is to understand the form pre-fill capabilities present within vets-api and how to set it up and utilize it for our specific forms (21-22 & 21-22a) regarding Appoint a Rep.

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Tasks:

• [x] Research Form Pre-Fill Functionality in vets-api for Forms 21-22 & 21-22a
• [x] Document in a comment below how to implement pre-fill for our 21-22 and 21-22a forms, also note anything we may need to consider outside of the defaults for pre-fill.

Acceptance Criteria:

• [x] We have a clear understanding of the pre-fill functionality and how it relates to our forms.
• [ ] We understand if we need to do anything outside of the defaults.

[holdenhinkle]

Here's the pre-fill documentation - https://depo-platform-documentation.scrollhelp.site/developer-docs/va-forms-library-how-to-work-with-pre-fill#VAFormsLibrary-HowtoworkwithPre-Fill-Vets-APIWork

[opticbob]

Form pre-fill for the vets-api backend is thoroughly explained here: https://github.com/department-of-veterans-affairs/va.gov-team/blob/master/teams/vsa/teams/ebenefits/engineering/prefill_setup.md. An example PR that runs through all the necessary additions is here: https://github.com/department-of-veterans-affairs/vets-api/pull/5391.

We will need:

• Two prefill classes, one each for 21-22 and 21-22a. These prefill classes need a metadata method that includes that we want to enable prefill, a version, and a return URL. The return URL is the first page of the form. Example: https://github.com/department-of-veterans-affairs/vets-api/blob/master/app/models/form_profiles/va_0993.rb.
• Two form profile mappings that define what fields we want prefilled and where that data comes from. Example: config/form_profile_mappings/28-1900.yml.
• Add the two forms to app/models/form_profile.rb. They need to be defined under ALL_FORMS and FORM_ID_TO_CLASS.
• Add a definition of our form with prefill: true to config/settings.yml
• Add two tests to spec/models/form_profile_spec.rb to cover each form

Outstanding questions:

1. Are our form URLs already determined?
2. Do we already have form names that map to our form numbers like vre_readiness: ['28-1900']?