chore(slack): move some floating functions into SlackService

[cathteng]

Move _get_attachments, _notify_recipient, and _send_message_to_slack_channel under SlackService, which were previously floating functions. We should centralize Slack logic where possible.

Also fixes typing for SlackBlock and SlackAttachment (which are literally the same thing), we should be using dict since we're mutating the result in get_attachments (typing is enforced in its home).

[codecov]

Codecov Report

Attention: Patch coverage is 93.97590% with 5 lines in your changes missing coverage. Please review.

Project coverage is 78.12%. Comparing base (6b1b5cd) to head (31c972e).
Report is 37 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff             @@
##           master   #73695       +/-   ##
===========================================
+ Coverage   56.99%   78.12%   +21.13%     
===========================================
  Files        6642     6651        +9     
  Lines      297290   297594      +304     
  Branches    51155    51204       +49     
===========================================
+ Hits       169446   232506    +63060     
+ Misses     123101    58777    -64324     
- Partials     4743     6311     +1568     

Files	Coverage Δ
...try/integrations/slack/message_builder/__init__.py	100.00% <100.00%> (ø)
...y/integrations/slack/message_builder/base/block.py	94.05% <100.00%> (+53.28%)	⬆️
src/sentry/integrations/slack/notifications.py	97.29% <100.00%> (+53.39%)	⬆️
...try/notifications/additional_attachment_manager.py	80.64% <100.00%> (+16.12%)	⬆️
...rc/sentry/tasks/integrations/slack/post_message.py	87.50% <60.00%> (+25.96%)	⬆️
src/sentry/integrations/slack/service.py	93.52% <95.31%> (+58.34%)	⬆️

... and 1993 files with indirect coverage changes