fix: update serializers for field constraints

[marlonkeating]

Description:
Follow-up for #916, with needed updates to serializers for checkout context bff field constraints.

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 full_name, admin_email, country, company_name

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]

good idea