setup and implementation of a body wrapper in otelmux

[martinyonatann]

setup and implementation of a body wrapper in the go.opentelemetry.io/contrib/instrumentation/github.com/gorilla/mux/otelmux package

This PR addresses feedback provided in #6648

[codecov]

Codecov Report

Attention: Patch coverage is 92.30769% with 9 lines in your changes missing coverage. Please review.

Project coverage is 68.8%. Comparing base (9a53a30) to head (0c1f634).
Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
...trumentation/github.com/gorilla/mux/otelmux/mux.go	76.6%	5 Missing and 2 partials ⚠️
...ub.com/gorilla/mux/otelmux/request/body_wrapper.go	93.7%	2 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##            main   #6650     +/-   ##
=======================================
+ Coverage   68.7%   68.8%   +0.1%     
=======================================
  Files        200     202      +2     
  Lines      16872   16958     +86     
=======================================
+ Hits       11601   11681     +80     
- Misses      4927    4932      +5     
- Partials     344     345      +1     

Files with missing lines	Coverage Δ
...gorilla/mux/otelmux/request/resp_writer_wrapper.go	100.0% <100.0%> (ø)
...ub.com/gorilla/mux/otelmux/request/body_wrapper.go	93.7% <93.7%> (ø)
...trumentation/github.com/gorilla/mux/otelmux/mux.go	85.0% <76.6%> (-4.3%)	⬇️

... and 1 file with indirect coverage changes