ref(alerts): Define anomaly detection connection pool once

[ceorourke]

We had seer_anomaly_detection_connection_pool defined in a few places, this just moves it to be defined once in utils.py and imports it as needed.

Some tests will fail until I can rebase on top of https://github.com/getsentry/sentry/pull/78652

[codecov[bot]]

:x: 4 Tests Failed:

Tests completed	Failed	Passed	Skipped
329	4	325	9

View the top 3 failed tests by shortest run time

 tests.sentry.incidents.endpoints.test_organization_alert_rule_anomalies

Stack Traces | 0s run time

No failure message available

 tests.sentry.incidents.endpoints.test_organization_alert_rule_details

Stack Traces | 0s run time

No failure message available

 tests.sentry.incidents.endpoints.test_organization_alert_rule_index

Stack Traces | 0s run time

No failure message available

To view individual test run time comparison to the main branch, go to the Test Analytics Dashboard

[getsantry[bot]]

This pull request has gone three weeks without activity. In another week, I will close it.

But! If you comment or otherwise update it, I will reset the clock, and if you add the label WIP, I will leave it alone unless WIP is removed ... forever!

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"A weed is but an unloved flower." ― Ella Wheeler Wilcox 🥀