Add github.com/julienschmidt/httprouter instrumentation

[MrAlias]

This new module instruments the github.com/julienschmidt/httprouter module.

• Comply with OpenTelemetry HTTP semantic conventions
• Package documentation
• Exported objects are documented
• Tests and linting are passing
• Includes dependabot config
• Library added to README.md
• CHANGELOG.md updated

[codecov-commenter]

Codecov Report

Merging #273 (55e64d7) into main (fc5ab1d) will increase coverage by 0.26%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #273      +/-   ##
==========================================
+ Coverage   77.01%   77.27%   +0.26%     
==========================================
  Files          47       48       +1     
  Lines        2149     2174      +25     
==========================================
+ Hits         1655     1680      +25     
  Misses        449      449              
  Partials       45       45              

Flag	Coverage Δ
Linux	77.27% <100.00%> (+0.26%)	⬆️
macOS	75.39% <100.00%> (+0.28%)	⬆️

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Impacted Files	Coverage Δ
...nschmidt/httprouter/splunkhttprouter/httprouter.go	100.00% <100.00%> (ø)

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