feat(crons): Add endpoint to return counts by status

[malwilley]

GET /organizations/<org_slug>/monitors-count/?project=1&environment=prod

{
  "counts": {
    "total": 4,
    "active": 3,
    "disabled": 1
  }
}

This endpoint is necessary because we want to display the number of active cron monitors from the issues page. Current endpoints are paginated and fetch more information than necessary.

LMK if this is not the right url or response format or if I'm doing anything else wrong. There doesn't seem to be a standard way of setting up count endpoints AFAICT.

[codecov]

Codecov Report

Attention: Patch coverage is 96.61017% with 2 lines in your changes missing coverage. Please review.

✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
...tors/endpoints/organization_monitor_index_count.py	93.75%	2 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##           master   #92574   +/-   ##
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  Coverage   87.89%   87.89%           
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  Files       10236    10238    +2     
  Lines      586985   587058   +73     
  Branches    22805    22805           
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+ Hits       515935   516002   +67     
- Misses      70620    70626    +6     
  Partials      430      430