chore(opentelemetry-otlp): Make invalid combinations of transport & encoding unrepresentable

[scottgerring]

Really A Draft: Trying a different approach; please hold off on reviews!

Fixes #3082
This PR removes the ability to specify invalid combinations of transport (HTTP/gRPC) and encoding (JSON/Protobuf) by making the invalid combinations unrepresentable.

For completeness, I staged a separate branch that simply errors out when a user specifies an invalid combination. This has less impact on the API surface and is a smaller change, but doesn't prevent the user from specifying the invalid combinations. I prefer this variant here where we use the type system!

Changes

We do this by:

• Removing with_protocol from the WithExportConfig shared between both gRPC and HTTP exporters
• Introducing HttpEncodingProtocol, and supporting methods on WithHttpConfig to configure JSON or Protobuf encoding

I wanted to draw a clear line between 'protocol' and 'encoding' as this feels like where the confusion has crept in, but the spec itself uses the term protocol to refer to both application layer protocol and body encoding.

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 93.97590% with 5 lines in your changes missing coverage. Please review.
✅ Project coverage is 80.7%. Comparing base (5c8dbc8) to head (829a910).

Files with missing lines	Patch %	Lines
opentelemetry-otlp/src/exporter/http/mod.rs	83.8%	5 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##            main   #3229     +/-   ##
=======================================
- Coverage   80.8%   80.7%   -0.1%     
=======================================
  Files        128     128             
  Lines      23156   23152      -4     
=======================================
- Hits       18711   18704      -7     
- Misses      4445    4448      +3     

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[bantonsson]

I don't see anything in this that can't be derived.

[scottgerring]

Good catch!