Add DigiCert configs. (#26780)

For #26603

This PR includes:
- DigiCert configs
- Some config code for custom SCEP, but not fully functional
- Validation tests for DigiCert Configs
- DigiCert activities (manually tested)

This PR does not include the following:
- Encryption of DigiCert API tokens -- this requires DB migration (in
next PR)
- Making an HTTP call to DigiCert API for validation (in later PR)
- Integration tests (in later PR)
- Changes file (in later PR)

# Checklist for submitter

- [x] Added/updated automated tests
- [x] Manual QA for all new/changed functionality

cmd/fleetctl/testdata/expectedGetConfigIncludeServerConfigJson.json

@@ -146,7 +146,9 @@
       "jira": null,
       "zendesk": null,
       "google_calendar": null,
-      "ndes_scep_proxy": null
+      "ndes_scep_proxy": null,
+      "custom_scep_proxy": null,
+      "digicert": null
     },
     "update_interval": {
       "osquery_detail": "1h0m0s",