Refactor the Duplicated code in OTLP Exporter with transform part

[hanyuancheung]

Fix: part of #2641
Follow: #2684

• Refactor otlpconfig part from exporter/otlp/otlpmetric(otlptrace)/internal/ into exporter/otlp/internal/envconfig
• Refactor transform part from exporter/otlp/otlpmetric(otlptrace)/internal/ into exporter/otlp/internal/transform
• Refactor connection part from exporter/otlp/otlpmetric(otlptrace)/internal/ exporter/otlp/internal/connection

[codecov]

Codecov Report

Merging #2685 (0640dd5) into main (70a5446) will decrease coverage by 0.0%.
The diff coverage is 90.0%.

@@           Coverage Diff           @@
##            main   #2685     +/-   ##
=======================================
- Coverage   76.7%   76.6%   -0.1%     
=======================================
  Files        181     179      -2     
  Lines      12175   12059    -116     
=======================================
- Hits        9345    9248     -97     
+ Misses      2605    2587     -18     
+ Partials     225     224      -1     

Impacted Files	Coverage Δ
exporters/otlp/internal/transform/attribute.go	100.0% <ø> (ø)
exporters/otlp/internal/transform/resource.go	100.0% <ø> (ø)
...otlp/otlpmetric/internal/metrictransform/metric.go	78.1% <83.3%> (ø)
...ers/otlp/otlptrace/internal/tracetransform/span.go	96.5% <100.0%> (ø)
attribute/set.go	63.2% <0.0%> (+1.3%)	⬆️
sdk/trace/batch_span_processor.go	82.1% <0.0%> (+1.8%)	⬆️
attribute/iterator.go	84.0% <0.0%> (+2.8%)	⬆️
attribute/kv.go	91.6% <0.0%> (+41.6%)	⬆️

[dmathieu]

This seems to encompass many more things other than the exporter refactor (there are several go.mod changed, and an unrelated changelog entry added).
How about splitting unrelated changes into separate PRs?

[hanyuancheung]

Close. Refactor in other PRs.