otelhttp: export SpanNameFormatters

[seankhliao]

Based on top of #7192

Makes SpanNameFormatter a type alias for documentation.
Exports the default formatters.
Adds a new formatter for use with http mux patterns.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 80.7%. Comparing base (a011722) to head (a44c60f).
Report is 7 commits behind head on main.

Additional details and impacted files

@@          Coverage Diff          @@
##            main   #7197   +/-   ##
=====================================
  Coverage   80.7%   80.7%           
=====================================
  Files        205     205           
  Lines      18083   18089    +6     
=====================================
+ Hits       14601   14613   +12     
+ Misses      3062    3058    -4     
+ Partials     420     418    -2     

Files with missing lines	Coverage Δ
instrumentation/net/http/otelhttp/config.go	85.0% <100.0%> (+1.2%)	⬆️
instrumentation/net/http/otelhttp/handler.go	92.9% <100.0%> (+<0.1%)	⬆️
instrumentation/net/http/otelhttp/transport.go	94.7% <100.0%> (-0.1%)	⬇️

... and 1 file with indirect coverage changes

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