feat: Add more checkout field constraints

[marlonkeating]

Description:
Adding more constraint fields for field validation to the checkout context BFF payload.

Jira:
ENT-10753

Testing Instructions

• Navigate to http://localhost:18270/api-docs/
• Scroll to v1/bffs/checkout/context/ POST entry
• Click "Try it out" -> Execute
• Verify the response field_constraints contains the new entries

Validation
Validate that regexes provide the proper constraints for the fields in question (Good tool: https://regex101.com/ )

• admin_email: enforces valid email format
• country: enforces capitalized letters

Merge checklist:

• ./manage.py makemigrations has been run
  • Note: This must be run if you modified any models.
    • It may or may not make a migration depending on exactly what you modified, but it should still be run.

Post merge:

• Ensure that your changes went out to the stage instance
• Deploy to prod instance

[iloveagent57]

looks great! one tiny request

[brobro10000]

Approved to unblock but as part of making validation complete, it requires a fix to company name in the abstract serializer in order to surface the errors downstream to the form fields.

In enterprise_access/apps/api/serializers/customer_billing.py, the CustomerBillingCreateCheckoutSessionValidationFailedResponseSerializer does not have company_name as as valid field abstracted. Adding the company name field as part of the serializer allows the downstream component to properly surface the correct error.

    company_name = FieldValidationSerializer(
        required=False,
        help_text='Validation results for company_name if validation failed. Absent otherwise.',
    )

It results in the downstream form field for company_name to properly surface the correct error (albeit not user friendly yet).