♻️ ref(slack): pull thread util methods into a separate class

[iamrajjoshi]

i will end up reusing these methods between issue and metric alerts, so pulling it out of the action handler into a generic util class we can reuse.

id imagine we probably delete this after notification platform. didn't make it super generic yet because of the rule of 3.

[eyoandpengu]

rule of 3 ? These util methods are only used for slack metric & issue alerts, those are the two strikes (?)

[Christinarlong]

I approved with my test account 💀

[codecov]

Codecov Report

Attention: Patch coverage is 76.92308% with 9 lines in your changes missing coverage. Please review.

✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
src/sentry/integrations/slack/utils/threads.py	75.00%	9 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #87302      +/-   ##
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- Coverage   87.73%   87.73%   -0.01%     
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  Files        9863     9864       +1     
  Lines      558646   558681      +35     
  Branches    22028    22028              
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+ Hits       490151   490181      +30     
- Misses      68088    68093       +5     
  Partials      407      407