feat(crons): Add api for fetching checking processing errors for a monitor.

[wedamija]

This allows us to return processing errors for monitors. Will follow up with apis to dismiss/delete items and to fetch errors at the project level too.

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 97.29730% with 1 lines in your changes are missing coverage. Please review.

Project coverage is 80.04%. Comparing base (07ac900) to head (e2d02f2).

Additional details and impacted files

@@           Coverage Diff           @@
##           master   #70557   +/-   ##
=======================================
  Coverage   80.04%   80.04%           
=======================================
  Files        6502     6503    +1     
  Lines      290534   290571   +37     
  Branches    50067    50073    +6     
=======================================
+ Hits       232547   232587   +40     
+ Misses      57549    57546    -3     
  Partials      438      438           

Files	Coverage Δ
src/sentry/api/urls.py	100.00% <100.00%> (ø)
...dpoints/project_monitor_processing_errors_index.py	100.00% <100.00%> (ø)
src/sentry/monitors/serializers.py	100.00% <100.00%> (ø)
src/sentry/monitors/processing_errors.py	97.74% <88.88%> (-0.65%)	:arrow_down:

... and 9 files with indirect coverage changes