🎲 random_once router type

[rowanseymour]

Re-adding from #788

[codecov]

Codecov Report

❌ Patch coverage is 64.51613% with 11 lines in your changes missing coverage. Please review.
✅ Project coverage is 86.49%. Comparing base (a5ffa94) to head (88c4648).
⚠️ Report is 1792 commits behind head on main.

Files with missing lines	Patch %	Lines
flows/routers/random_once.go	64.51%	7 Missing and 4 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #888      +/-   ##
==========================================
- Coverage   86.56%   86.49%   -0.07%     
==========================================
  Files         237      238       +1     
  Lines       10403    10434      +31     
==========================================
+ Hits         9005     9025      +20     
- Misses        973      980       +7     
- Partials      425      429       +4     

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