Split of OTLP package including making more of interface public to enable more use cases

[thompson-tomo]

Feature Request

Is your feature request related to a problem?

I want to be log my metrics to file using the same schema as used when transmitted to the cloud and have the flexibility to use a different transport ie RabbitMQ.

Describe the solution you'd like:

I would like the cureent OpenTelemetry.Exporter.OpenTelemetryProtocol package to be split and existing package to become a meta package. Split would be as follows:

• Schema classes are moved into OpenTelemetry.Exporter.OpenTelemetryProtocol.Schema
• GRPC transport implementation is moved into OpenTelemetry.Exporter.OpenTelemetryProtocol.GRPC
• Http transport implementation is moved into OpenTelemetry.Exporter.OpenTelemetryProtocol.Http
• Common Functionality ie Enum is moved into OpenTelemetry.Exporter.OpenTelemetryProtocol.Abstractions

Describe alternatives you've considered.

Generate own classes based on proto files

Additional Context

n/a