chore: add slack notification actions for new and stale PRs

[artemijspavlovs]

PR Standards

Opening a pull request should be able to meet the following requirements

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For Author:

• Targeted PR against correct branch
• included the correct type prefix in the PR title
• Linked to Github issue with discussion and accepted design
• Targets only one github issue
• Wrote unit and integration tests
• All CI checks have passed
• Added relevant godoc comments

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For Reviewer:

• confirmed the correct type prefix in the PR title
• Reviewers assigned
• confirmed all author checklist items have been addressed

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After reviewer approval:

• In case targets main branch, PR should be squashed and merged.
• In case PR targets a release branch, PR should be rebased.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 44.18%. Comparing base (74e7a92) to head (5b9e9b4).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #613   +/-   ##
=======================================
  Coverage   44.18%   44.18%           
=======================================
  Files         119      119           
  Lines       16474    16474           
=======================================
  Hits         7279     7279           
  Misses       8168     8168           
  Partials     1027     1027           

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