Add IdGenerator in tracer_provider

[marcalff]

The specification defines IdGenerator as an extension point:

https://github.com/open-telemetry/opentelemetry-specification/blob/main/specification/trace/sdk.md#id-generators

The yaml for tracer_provider: should probably allow to define which id generator to use, something like:

tracer_provider:
  id_generator:
    # SDK extension point
    my_custom_id_generator:

Found while prototyping for opentelemetry-cpp

[jack-berg]

This makes sense 👍

[jack-berg]

Hey @marcalff - marked this as "not blocking stability" because I think we can safely add this after stable if needed. But I think if you (or anyone) wants to take this now, we'd welcome PRs.