Add automation_type filter to count policies endpoint

[jkatz01]

Related issue: Resolves #41987

Checklist for submitter

Testing

• Added/updated automated tests

• Where appropriate, automated tests simulate multiple hosts and test for host isolation (updates to one hosts's records do not affect another)

• QA'd all new/changed functionality manually

  • Tested with the "scripts" filter and >20 policies with that automation, and together with Fleet UI: Unreleased bug fixes for policy automations filtering #41991 the policy count and pagination is correct

[codecov]

Codecov Report

❌ Patch coverage is 80.00000% with 6 lines in your changes missing coverage. Please review.
✅ Project coverage is 66.43%. Comparing base (c4d142a) to head (44194d3).
⚠️ Report is 53 commits behind head on main.

Files with missing lines	Patch %	Lines
server/datastore/mysql/policies.go	75.00%	3 Missing and 3 partials ⚠️

Additional details and impacted files

@@           Coverage Diff            @@
##             main   #42007    +/-   ##
========================================
  Coverage   66.42%   66.43%            
========================================
  Files        2511     2512     +1     
  Lines      200786   201435   +649     
  Branches     9059     9059            
========================================
+ Hits       133363   133814   +451     
- Misses      55367    55532   +165     
- Partials    12056    12089    +33     

Flag	Coverage Δ
backend	68.21% <80.00%> (+<0.01%)	⬆️

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