Impossible to apply SessionIdRatioBasedSampler after session refactoring

[GerardPaligot]

Now that we should use SessionManager to generate the session id and because ServiceManager getServiceManager from OpenTelemetryRumBuilder or SessionManagerImpl aren't public, it is impossible to call setSampler from SdkTracerProviderBuilder to initialize SessionIdRatioBasedSampler class.

otelRumBuilder.addTracerProviderCustomizer { tracerProvider, _ ->
  return@addTracerProviderCustomizer tracerProvider
    .setSampler(SessionIdRatioBasedSampler(ratio, /* session provider here but we can't get it */))
}

[LikeTheSalad]

cc @breedx-splk @surbhiia @bidetofevil

This sounds related to what we discussed in the last SIG meeting, in which we should allow the session manager to be retrieved from OTel RUM. Then we would be able to enable a lot of use cases by providing the OTel RUM object, including the automatic instrumentation that needs access to the session manager to work.