Refactor: separate Eligiblity API config from EnrollmentFlow

[thekaveman]

Once #2721 is complete, we'll have refactored all of the claims verification related fields out of the EnrollmentFlow model (using ClaimsVerificationRequest in cdt_identity instead).

We want to undertake a similar effort to refactor an ApiVerificationRequest model out of EnrollmentFlow. This is part of our longer-term effort to decompose the Benefits app into smaller, more re-usable pieces.

The new model should capture the following fields from EnrollmentFlow:

eligibility_api_url 
eligibility_api_auth_header
eligibility_api_auth_key_secret_name
eligibility_api_public_key
eligibility_api_jwe_cek_enc
eligibility_api_jwe_encryption_alg
eligibility_api_jws_signing_alg
eligibility_form_class

Then it should be referenced back on EnrollmentFlow as an (optional) foreign key field:

api_request = models.ForeignKey(
    ApiVerificationRequest,
    on_delete=models.PROTECT,
    null=True,
    blank=True,
)

Acceptance Criteria

• Eligibility API configuration from EnrollmentFlow is moved to its own model class
• EnrollmentFlow maintains a reference to the new model
• A data migration maintains existing data
• Updated permissions to match existing EnrollmentFlow permissions for these fields