Implement custom protoc plugin to generate OTLP JSON class definitions

[herin049]

Description

This PR implements a custom protoc plugin to automatically generate OTLP JSON class definitions. Consensus was reached deciding that a custom protoc plugin would be the best route for adding support for OTLP JSON while minimizing the number of runtime dependencies (for additional context reference #4886). This PR is a refined version of the first part of a set of changes that are in #4902. Please view the draft PR for an overview of how these packages will be used.

Fixes #1003

Type of change

Please delete options that are not relevant.

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

How Has This Been Tested?

tox run -e $(tox --listenvs | grep opentelemetry-protojson | tr '\n' ',')

Does This PR Require a Contrib Repo Change?

• Yes. - Link to PR:
• No.

Checklist:

• Followed the style guidelines of this project
• Changelogs have been updated
• Unit tests have been added
• Documentation has been updated

[mmanciop]

I am very much looking forward to this landing: protobuf is a pretty toxic dependency when automatically injecting Python SDKs with the OpenTelemetry Injector