chore (anomaly detection): create proxy API endpoint

[mifu67]

Create a proxy API endpoint for us to query the backend about anomalies. get_historical_anomaly_data_from_seer returns a placeholder.

[nhsiehgit]

bunch of non blocking nits

[nhsiehgit]

nit: maybe lets add a note here to call out None is only hit when the seer response is bad
but if there are no anomalies, we'll get an empty array?

[mifu67]

If there are fewer than seven days of event data, return an empty array. Otherwise, we get an array of labeled "anomalies" (these are potential anomalies that have been evaluated by Seer)

[codecov]

Codecov Report

Attention: Patch coverage is 90.58824% with 8 lines in your changes missing coverage. Please review.

✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
...nts/endpoints/organization_alert_rule_anomalies.py	84.61%	3 Missing and 3 partials ⚠️
src/sentry/seer/anomaly_detection/utils.py	93.93%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #76934      +/-   ##
==========================================
- Coverage   78.22%   78.22%   -0.01%     
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  Files        6898     6900       +2     
  Lines      306690   306747      +57     
  Branches    50276    50283       +7     
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+ Hits       239913   239955      +42     
- Misses      60415    60429      +14     
- Partials     6362     6363       +1