feat(contrib-test-utils): copy soon-to-be-removed types from @opentelemetry/otlp-transformer

[pichlermarc]

Which problem is this PR solving?

We're about to remove the intermediate OTLP types from @opentelemetry/otlp-transformer as we now have an interface that allows us to serialize directly to OTLP(protobuf) or OTLP(json).

Since they were used for testing here, we need to duplicate the relevant ones so that we're not blocked on the next core update.

Closes #2565

Short description of the changes

• changes export * to explicit exports to avoid exporting anything we don't intend
• copies types from @opentelemetry/otlp-transformer
• moves the OtlpSpanKind enum from test-fixtures.ts to otlp-types.ts

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 90.75%. Comparing base (e3c9150) to head (73fe3f7).
Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2573      +/-   ##
==========================================
- Coverage   90.80%   90.75%   -0.05%     
==========================================
  Files         169      169              
  Lines        8026     8026              
  Branches     1635     1635              
==========================================
- Hits         7288     7284       -4     
- Misses        738      742       +4     

see 1 file with indirect coverage changes

[pichlermarc]

Note: TAV tests will fail here due to #2572 - it's unrelated to this PR.