For #26622
This PR includes:
- Making DigiCert client a real service that can be overridden in tests
- GitOps support for DigiCert and Custom SCEP configs
# Checklist for submitter
- [x] Added/updated automated tests
- [x] A detailed QA plan exists on the associated ticket (if it isn't
there, work with the product group's QA engineer to add it)
- [x] Manual QA for all new/changed functionality
