feat: Support mixed categories for semconv opt-in

[prestonvasquez]

Resolves #7246

Introducing support for category-specific values in the OTEL_SEMCONV_STABILITY_OPT_IN environment variable for otelhttp and otelmongo. For example, multiple categories should be supported without error:

export OTEL_SEMCONV_STABILITY_OPT_IN=http,database,messaging/dup

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 81.4%. Comparing base (a35d2d8) to head (ae167e0).
Report is 1 commits behind head on main.

Additional details and impacted files

@@          Coverage Diff          @@
##            main   #7274   +/-   ##
=====================================
  Coverage   81.4%   81.4%           
=====================================
  Files        206     206           
  Lines      18230   18270   +40     
=====================================
+ Hits       14848   14885   +37     
- Misses      2961    2963    +2     
- Partials     421     422    +1     

Files with missing lines	Coverage Δ
...lei/go-restful/otelrestful/internal/semconv/env.go	94.9% <100.0%> (+0.1%)	⬆️
....com/gin-gonic/gin/otelgin/internal/semconv/env.go	94.9% <100.0%> (+0.1%)	⬆️
...ub.com/gorilla/mux/otelmux/internal/semconv/env.go	93.0% <100.0%> (+0.2%)	⬆️
.../mongo/otelmongo/internal/semconv/event_monitor.go	100.0% <100.0%> (ø)
...tp/httptrace/otelhttptrace/internal/semconv/env.go	90.5% <100.0%> (+0.3%)	⬆️
...entation/net/http/otelhttp/internal/semconv/env.go	90.5% <100.0%> (+0.3%)	⬆️

... and 1 file with indirect coverage changes

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