R4R: added slack notification to runsim

[alessio]

• Linked to github-issue with discussion and accepted design OR link to spec that describes this work.
• Wrote tests
• Updated relevant documentation (docs/)
• Added a relevant changelog entry: clog add [section] [stanza] [message]
• rereviewed Files changed in the github PR explorer

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For Admin Use:

• Added appropriate labels to PR (ex. wip, ready-for-review, docs)
• Reviewers Assigned
• Squashed all commits, uses message "Merge pull request #XYZ: [title]" (coding standards)

[codecov]

Codecov Report

Merging #4547 into master will increase coverage by 0.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #4547      +/-   ##
==========================================
+ Coverage   53.62%   53.63%   +0.01%     
==========================================
  Files         260      260              
  Lines       16177    16177              
==========================================
+ Hits         8675     8677       +2     
+ Misses       6856     6854       -2     
  Partials      646      646