feat: make SignalGroupDescriptor generic

[tlambert03]

This PR is carrying on what I tried to do in #291.

It all works, but makes for an unappealing consequence when annotating with ClassVar[SignalGroupDescriptor] as described in #291 (comment)

working on it here

[codspeed-hq]

CodSpeed Performance Report

Merging #297 will not alter performance

_{Comparing tlambert03:generic-sig-group-decriptor (50ab2f6) with main (8bf2d82)}

Summary

✅ 67 untouched benchmarks

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.83%. Comparing base (8bf2d82) to head (50ab2f6).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #297   +/-   ##
=======================================
  Coverage   99.82%   99.83%           
=======================================
  Files          22       22           
  Lines        2351     2353    +2     
=======================================
+ Hits         2347     2349    +2     
  Misses          4        4           

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