Update minsev to allow dynamic severities

[MrAlias]

Similar to slog, allow the minsev.LogProcessor to be configured with dynamic severity resolution.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 65.8%. Comparing base (e1758b2) to head (b36db11).
Report is 1 commits behind head on main.

Additional details and impacted files

@@          Coverage Diff          @@
##            main   #6116   +/-   ##
=====================================
  Coverage   65.7%   65.8%           
=====================================
  Files        204     205    +1     
  Lines      13088   13097    +9     
=====================================
+ Hits        8610    8618    +8     
  Misses      4215    4215           
- Partials     263     264    +1     

Files with missing lines	Coverage Δ
processors/minsev/minsev.go	92.0% <100.0%> (+5.6%)	⬆️
processors/minsev/severity.go	100.0% <100.0%> (ø)

... and 1 file with indirect coverage changes

Needs rebasing and addressing dmathieu comments