Provide extensibility of the SDK to support vendor specific ID Generators

[wilguo]

Hi everyone,

I'm currently working on an ID Generator to generate traceIDs in order to support sending traces to AWS X-Ray. The ID Generator will generate a traceID based on the AWS X-Ray Trace ID Format. I submitted a PR for the AWS X-Ray ID Generator and one of the issues that came up is the IDGenerator interface is currently private which resulted in me copying over the interface.

Would it be possible to extend the SDK and expose the IDGenerator in order to allow vendor specific ID Generators to be implemented?

Thank you!

cc @alolita @rakyll