Update data payload for next steps email endpoint

[opticbob]

Background

There are some validation issues with the interplay between the frontend and the backend for the next steps email endpoint. We are going to move from sending a rep name, address, and type to sending a entity_type and entity_id so those values can be looked up on the backend. This will make it easier to cover both selections of individuals and orgs as a whole. The frontend also needs to send the email_address field.

Tasks

• Remove representative_address, _name, and _type fields.
• Add entity_type that should be validated to be individual or organization.
• Add entity_id.
• Add email_address. If there is a claimant email it should be that one, if not we should use the veteran's email.

Acceptance Criteria

• The frontend will generate a payload that return a 200 status from the backend.

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How to configure this issue

1. Label with practice area (backend, frontend,ux, design, research)
2. Label for MVP work? (mvp)
3. Add an estimate
4. Attach an Epic (what body of work is this a part of?)

[holdenhinkle]

PR - department-of-veterans-affairs/vets-website#32886