fix(slack): add filtering for users affected by env if the rule has an env

[cathteng]

Currently, we always fetch Users Affected across all environments, even if the rule has a non-null environment_id, which means the rule fires only if the event happened in a specific environment.

There is already a function in which we can pass environment_ids to Snuba to get the count of users affected for specific envs, but we've been passing None by default. We can change this to pass the environment_id of the rule(s) that fired the issue alert (this is somehow an array).

I also added an enum for the different kinds of context fields that can appear in a Slack notification (NotificationContextField), so we can use that instead of strings.

[cathteng]

this logic is messy, but we remove Users Affected if possible because it requires a Snuba query

[cathteng]

this is also messy and i'm not sure if there's a better way to do this without making every other function in SUPPORTED_CONTEXT_DATA take in an unused rules argument

[codecov]

Codecov Report

Attention: Patch coverage is 94.11765% with 2 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
...entry/integrations/slack/message_builder/issues.py	90.90%	0 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #80451      +/-   ##
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- Coverage   78.33%   78.33%   -0.01%     
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  Files        7206     7206              
  Lines      318682   318693      +11     
  Branches    43929    43932       +3     
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- Hits       249641   249634       -7     
- Misses      62677    62690      +13     
- Partials     6364     6369       +5