fix(aci): Add an initial UptimeSubscription serializer

[kcons]

Add UptimeSubscriptionSerializer to allow Detector endpoints to return successfully.
Detector data source subscription types need to be serializable for the detectors APIs.
Add a test to verify that we can return a serialized uptime detector; the specifics of the fields serialized are more a question for uptime folks and not yet relevant, so the serialization result contents are left untested.

[saponifi3d]

lgtm - cc @wedamija incase you have any thoughts on this (it's just meant to unblock some other dev)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

⚠️ Parser warning

The parser emitted a warning. Please review your JUnit XML file:

Warning while parsing testcase attributes: Limit of string is 1000 chars, for name, we got 2083 at 1:156718 in /home/runner/work/sentry/sentry/.artifacts/pytest.junit.xml

Additional details and impacted files

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##           master   #92150      +/-   ##
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- Coverage   85.56%   85.56%   -0.01%     
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  Files       10179    10179              
  Lines      583480   583539      +59     
  Branches    22627    22627              
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+ Hits       499254   499304      +50     
- Misses      83631    83640       +9     
  Partials      595      595