Add custom SCEP configs

[getvictor]

For #26603

This PR includes:

• Refactoring of NDES/SCEP verify/timeout logic for easier testing (with dependency injection)
• Custom SCEP configs
• saving/deleting/updating of encrypted custom SCEP challenges
• validation call to custom SCEP server to verify connection
• Custom SCEP activities
• unit and integration tests for all of the above

This PR does not include the following:

• Changes file (in later PR)

Checklist for submitter

• Added/updated automated tests
• Manual QA for all new/changed functionality

[codecov]

Codecov Report

Attention: Patch coverage is 91.74757% with 17 lines in your changes missing coverage. Please review.

Project coverage is 63.89%. Comparing base (de64d1f) to head (f6b4faf).
Report is 3 commits behind head on main.

Files with missing lines	Patch %	Lines
server/service/appconfig.go	90.55%	8 Missing and 4 partials ⚠️
cmd/fleet/serve.go	0.00%	2 Missing ⚠️
ee/server/service/scep_proxy.go	96.15%	1 Missing ⚠️
server/datastore/mysql/app_configs.go	95.65%	1 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   63.78%   63.89%   +0.10%     
==========================================
  Files        1726     1712      -14     
  Lines      163926   163654     -272     
  Branches     4366     4301      -65     
==========================================
- Hits       104563   104560       -3     
+ Misses      51216    50944     -272     
- Partials     8147     8150       +3     

Flag	Coverage Δ
backend	64.63% <91.74%> (+0.04%)	⬆️

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