[otlp-transformer] introduce separate entry-points for OTLP serializers

[pichlermarc]

Description

In preparation of marking @opentelemetry/otlp-transformer as stable (#4582) we should introduce separate entry-points to ensure that

• @experimental types/functions don't become part of the stable interface
  • use an /experimental entrypoint for this
• auto-generated protobuf code is not required from exporters that only want to use JSON (this has caused issues in the past, see @opentelemetry/otlp-transformer throw error when run in old Android webview, throw error "Cannot redefine property: name" #5096, use of eval is strongly discouraged #4987)

This issue is considered done when

• Experimental types/functions are available via @opentelemetry/otlp-transformer/experimental
• JSON and Protobuf serializers can be required from seperate entrypoints
  -example: @opentelemetry/otlp-transformer/json, @opentelemetry/otlp-transformer/protobuf
• entrypoints and stability expectations for each entrypoint have been documented in README.md