ref(mediators): Convert external issues mediators to dataclasses

[Christinarlong]

[x] Creator -> ExternalIssueCreator
[x] IssueLinkCreator
[x] tests
[x] typing

issue ref (#73857)

[codecov]

Codecov Report

Attention: Patch coverage is 98.33333% with 1 line in your changes missing coverage. Please review.

✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
.../sentry_apps/external_issues/issue_link_creator.py	97.36%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##           master   #78666   +/-   ##
=======================================
  Coverage   78.13%   78.13%           
=======================================
  Files        7103     7102    -1     
  Lines      312864   312864           
  Branches    51092    51096    +4     
=======================================
+ Hits       244458   244460    +2     
  Misses      62037    62037           
+ Partials     6369     6367    -2