Add 'otel' preset to severity parser

[djaglowski]

This preset mapping recognizes severity levels in native OpenTelemetry format, including both numeric and string representations.

Resolves #456

[codecov[bot]]

Codecov Report

Merging #460 (310a8f8) into main (8192584) will increase coverage by 0.2%. The diff coverage is 100.0%.

@@           Coverage Diff           @@
##            main    #460     +/-   ##
=======================================
+ Coverage   76.1%   76.4%   +0.2%     
=======================================
  Files         83      83             
  Lines       4016    4066     +50     
=======================================
+ Hits        3060    3107     +47     
- Misses       671     674      +3     
  Partials     285     285             

Impacted Files	Coverage Δ
operator/helper/severity_builder.go	90.3% <100.0%> (+4.6%)	:arrow_up:
operator/transformer/recombine/recombine.go	75.6% <0.0%> (-2.1%)	:arrow_down: