Add semconvutil and use it instead of httpconv and netconv

[pellared]

Why

Towards open-telemetry/opentelemetry-go#4081 (point 1)

Closes open-telemetry/opentelemetry-go#3744

It will make updating semconv in https://github.com/open-telemetry/opentelemetry-go a lot easier. This repo will only contain the "currently used" semconv code. Updating to newer will also be easier as we will change existing code instead of generating new code.

What

1. Add semconvutil as a internal shared package which is not to be published (Go templates).
2. Copy semconvutil package (via go generate + gotmpl) and use it instead of httpconv and netconv packages which are planned to be deprecated.

The "copy package" via go generate approach is used in order to not introduce dependencies between modules. More: open-telemetry/opentelemetry-go#3846

[codecov]

Codecov Report

Merging #3817 (5b8181c) into main (849072e) will increase coverage by 9.2%.
The diff coverage is 100.0%.

Additional details and impacted files

@@           Coverage Diff           @@
##            main   #3817     +/-   ##
=======================================
+ Coverage   70.1%   79.3%   +9.2%     
=======================================
  Files        149     163     +14     
  Lines       7133   10325   +3192     
=======================================
+ Hits        5004    8196   +3192     
  Misses      1998    1998             
  Partials     131     131             

Impacted Files	Coverage Δ
...stful/otelrestful/internal/semconvutil/httpconv.go	100.0% <100.0%> (ø)
...estful/otelrestful/internal/semconvutil/netconv.go	100.0% <100.0%> (ø)
...hub.com/emicklei/go-restful/otelrestful/restful.go	100.0% <100.0%> (ø)
...ation/github.com/gin-gonic/gin/otelgin/gintrace.go	83.5% <100.0%> (ø)
...gonic/gin/otelgin/internal/semconvutil/httpconv.go	100.0% <100.0%> (ø)
...-gonic/gin/otelgin/internal/semconvutil/netconv.go	100.0% <100.0%> (ø)
...rilla/mux/otelmux/internal/semconvutil/httpconv.go	100.0% <100.0%> (ø)
...orilla/mux/otelmux/internal/semconvutil/netconv.go	100.0% <100.0%> (ø)
...trumentation/github.com/gorilla/mux/otelmux/mux.go	88.4% <100.0%> (ø)
...entation/github.com/labstack/echo/otelecho/echo.go	100.0% <100.0%> (ø)
... and 12 more