Decouple otlp/otlptrace/internal from otlp/internal using gotmpl

[MrAlias]

Currently the otlp exporters share internal packages across package boundaries. This is something we want to stop doing: #3846. This PR addresses the cross-module internal package use for the otel/exporters/otlp/otlptrace package (but not its sub-modules).

• Add the gotmpl tooling to the project
• Add an otel/internal/shared package to hold all templates rendered by the gotmpl tooling
  • Add the otel/exporters/otlp/internal/retry package to the shared templates
  • Add the otel/exporters/otlp/internal/envconfig package to the shared templates
  • Add the otel/exporters/otlp/otlptrace/internal package to the shared templates
    • These are used here, but they are also expected to be used in the following PRs
• Use gotmpl to render the new retry and envconfig packages in otel/exporters/otlp/otlptrace/internal
• Use gotmpl to render the rest of otel/exporters/otlp/otlptrace/internal so it can use the now local retry and envconfig packages.
• Update otlptracegrpc and otlptracehttp to use the otel/exporters/otlp/otlptrace/internal packages

What is not included

Both the otlptracegrpc and otlptracehttp packages can now also have copies of these (and other) packages. This will decouple both packages from the otlptrace/internal packages.

The plan once this is merged is to tackle each of those packages in their own PRs.

Why not skip otlptrace/internal and just do everything in otlptracegrpc/internal and otlptracehttp/internal?

The otlptrace/internal package is planned to be flattened into both. However, for backwards compatibility reasons otlptrace/internal cannot be removed even after the sub-modules no longer depend on it.

Given otlptrace/internal will need to exist in the future until a major version bump, it is updated here to at least not have a cross module internal package dependency (other than otel/internal/global).

[codecov]

Codecov Report

Merging #4397 (40215d1) into main (c4d2d7c) will increase coverage by 0.0%.
The diff coverage is 91.2%.

Additional details and impacted files

@@          Coverage Diff           @@
##            main   #4397    +/-   ##
======================================
  Coverage   83.5%   83.6%            
======================================
  Files        184     186     +2     
  Lines      14448   14629   +181     
======================================
+ Hits       12073   12238   +165     
- Misses      2145    2157    +12     
- Partials     230     234     +4     

Files Changed	Coverage Δ
exporters/otlp/otlptrace/internal/header.go	100.0% <ø> (ø)
...rs/otlp/otlptrace/internal/otlpconfig/envconfig.go	89.5% <ø> (ø)
...xporters/otlp/otlptrace/internal/otlpconfig/tls.go	62.5% <ø> (ø)
...tlp/otlptrace/internal/tracetransform/attribute.go	100.0% <ø> (ø)
...lptrace/internal/tracetransform/instrumentation.go	100.0% <ø> (ø)
...otlp/otlptrace/internal/tracetransform/resource.go	100.0% <ø> (ø)
...ers/otlp/otlptrace/internal/tracetransform/span.go	96.5% <ø> (ø)
exporters/otlp/otlptrace/otlptracegrpc/client.go	92.5% <ø> (ø)
exporters/otlp/otlptrace/otlptracegrpc/options.go	76.4% <ø> (ø)
exporters/otlp/otlptrace/otlptracehttp/client.go	77.5% <ø> (ø)
... and 4 more

... and 2 files with indirect coverage changes