chore(otlp): prevent invalid combinations of exporter and protocol

[scottgerring]

Fixes #3082
Fixes #3208

Changes

Make invalid combinations of configured exporter features and exporter protocols unrepresentable by using feature flags, and adds support for OTEL_EXPORTER_OTLP_PROTOCOL

Alternative to #3229. In #3229, I've taken the extra step of moving the configurable protocol on the builders down into the HTTP client only, as it is not actually configurable on the tonic exporter - there's no choice.

After speaking to @bantonsson I reckon this is a lighter touch change which feels appropriate.

Please provide a brief description of the changes here.

Merge requirement checklist

• CONTRIBUTING guidelines followed
• Unit tests added/updated (if applicable)
• Appropriate CHANGELOG.md files updated for non-trivial, user-facing changes
• Changes in public API reviewed (if applicable)

[codecov]

Codecov Report

❌ Patch coverage is 89.47368% with 6 lines in your changes missing coverage. Please review.
✅ Project coverage is 80.8%. Comparing base (5c8dbc8) to head (3d785f6).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
opentelemetry-otlp/src/exporter/http/mod.rs	57.1%	3 Missing ⚠️
opentelemetry-otlp/src/exporter/http/logs.rs	50.0%	1 Missing ⚠️
opentelemetry-otlp/src/exporter/http/metrics.rs	50.0%	1 Missing ⚠️
opentelemetry-otlp/src/exporter/http/trace.rs	50.0%	1 Missing ⚠️

Additional details and impacted files

@@          Coverage Diff          @@
##            main   #3230   +/-   ##
=====================================
  Coverage   80.8%   80.8%           
=====================================
  Files        128     129    +1     
  Lines      23156   23199   +43     
=====================================
+ Hits       18711   18749   +38     
- Misses      4445    4450    +5     

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[scottgerring]

@bantonsson , as you looked at the Bigger One, if you have a second!