otelconf: refactor resource kv code

[codeboten]

Non-functional refactor, moves common code to an internal package to remove need to replicate the code again with 1.0.0 implementation.

Part of splitting #8026

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 79.4%. Comparing base (99b9e2a) to head (7451291).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##            main   #8053     +/-   ##
=======================================
- Coverage   79.5%   79.4%   -0.2%     
=======================================
  Files        186     187      +1     
  Lines      11865   11833     -32     
=======================================
- Hits        9441    9401     -40     
- Misses      2070    2077      +7     
- Partials     354     355      +1     

Files with missing lines	Coverage Δ
otelconf/internal/kv/keyvalue.go	100.0% <100.0%> (ø)
otelconf/v0.2.0/resource.go	100.0% <100.0%> (ø)
otelconf/v0.3.0/resource.go	100.0% <100.0%> (ø)

... and 2 files with indirect coverage changes

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