feat(uptime): Enable uptime in self-hosted

[evanpurkhiser]

This brings uptime features self hosted sentry installations

This was previously blocked on docker images for the uptime-checker being available.

There is a feature flag and option that need to be enabled for now. But we'll likely remove these in the future.

Fixes #3304

A few notes

• We may want to expose an official way to disable internal-ip filtering in the uptime-checker. Since self-hosted users may wish to monitor internal services, and it's unclear that we filter those out.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.45%. Comparing base (bf660f3) to head (6d90d8b).
Report is 6 commits behind head on master.

✅ All tests successful. No failed tests found.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #3787   +/-   ##
=======================================
  Coverage   99.45%   99.45%           
=======================================
  Files           3        3           
  Lines         183      183           
=======================================
  Hits          182      182           
  Misses          1        1           

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