otelconf: add unmarshalers for otlp exporters

[codeboten]

Adding unmarshalers and tests for otlp http/grpc exporters. Also added a missing test for createHeaders.

Part of splitting #8026

[codecov]

Codecov Report

❌ Patch coverage is 91.66667% with 10 lines in your changes missing coverage. Please review.
✅ Project coverage is 80.9%. Comparing base (ccac382) to head (288f130).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
otelconf/config_json.go	88.0%	4 Missing and 4 partials ⚠️
otelconf/config_common.go	66.6%	2 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##            main   #8112     +/-   ##
=======================================
+ Coverage   80.8%   80.9%   +0.1%     
=======================================
  Files        194     194             
  Lines      12965   13073    +108     
=======================================
+ Hits       10486   10588    +102     
- Misses      2105    2107      +2     
- Partials     374     378      +4     

Files with missing lines	Coverage Δ
otelconf/config_yaml.go	98.6% <100.0%> (+0.5%)	⬆️
otelconf/config_common.go	87.5% <66.6%> (+1.7%)	⬆️
otelconf/config_json.go	94.5% <88.0%> (-2.0%)	⬇️

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