Add custom SCEP configs

[getvictor]

For #26603

Checklist for submitter

If some of the following don't apply, delete the relevant line.

• Changes file added for user-visible changes in changes/, orbit/changes/ or ee/fleetd-chrome/changes.
  See Changes files for more information.
• Input data is properly validated, SELECT * is avoided, SQL injection is prevented (using placeholders for values in statements)
• Added support on fleet's osquery simulator cmd/osquery-perf for new osquery data ingestion features.
• If paths of existing endpoints are modified without backwards compatibility, checked the frontend/CLI for any necessary changes
• If database migrations are included, checked table schema to confirm autoupdate
• For database migrations:
  • Checked schema for all modified table for columns that will auto-update timestamps during migration.
  • Confirmed that updating the timestamps is acceptable, and will not cause unwanted side effects.
  • Ensured the correct collation is explicitly set for character columns (COLLATE utf8mb4_unicode_ci).
• Added/updated automated tests
• 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)
• Manual QA for all new/changed functionality
• For Orbit and Fleet Desktop changes:
  • Make sure fleetd is compatible with the latest released version of Fleet (see Must rule).
  • Orbit runs on macOS, Linux and Windows. Check if the orbit feature/bugfix should only apply to one platform (runtime.GOOS).
  • Manual QA must be performed in the three main OSs, macOS, Windows and Linux.
  • Auto-update manual QA, from released version of component to new version (see tools/tuf/test).
• For unreleased bug fixes in a release candidate, confirmed that the fix is not expected to adversely impact load test results or alerted the release DRI if additional load testing is needed.

[codecov]

Codecov Report

Attention: Patch coverage is 84.61538% with 4 lines in your changes missing coverage. Please review.

Project coverage is 63.99%. Comparing base (b91e8eb) to head (09a78bd).

Files with missing lines	Patch %	Lines
server/service/appconfig.go	75.00%	3 Missing ⚠️
server/datastore/mysql/ca_config_assets.go	85.71%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #27045   +/-   ##
=======================================
  Coverage   63.99%   63.99%           
=======================================
  Files        1706     1706           
  Lines      162934   162930    -4     
  Branches     4310     4310           
=======================================
  Hits       104273   104273           
+ Misses      50536    50533    -3     
+ Partials     8125     8124    -1     

Flag	Coverage Δ
backend	64.76% <84.61%> (+<0.01%)	⬆️

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