feat(default detector): create default detector for new projects

[shayna-ch]

create a default metric monitor for new projects

with these settings:

• Default assignee is the team that is assigned to the project
• Defaults to event.type is error
• Enabled on creation
• 15 minute interval, low sensitivity, above bounds only

Feature Flag organizations:default-anomaly-detector

[codecov[bot]]

Codecov Report

:x: Patch coverage is 89.83051% with 6 lines in your changes missing coverage. Please review. :white_check_mark: All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
src/sentry/receivers/project_detectors.py	80.00%	4 Missing :warning:
src/sentry/workflow_engine/processors/detector.py	94.73%	2 Missing :warning:

Additional details and impacted files

@@             Coverage Diff             @@
##           master   #104208      +/-   ##
===========================================
+ Coverage   80.50%    80.61%   +0.11%     
===========================================
  Files        9342      9344       +2     
  Lines      399240    400731    +1491     
  Branches    25560     25560              
===========================================
+ Hits       321408    323056    +1648     
+ Misses      77383     77226     -157     
  Partials      449       449