Support using the new composable samplers for the newer consistent sampling, e.g.:

import {
  createCompositeSampler,
  createComposableAlwaysOffSampler,
  createComposableAlwaysOnSampler,
  createComposableParentThresholdSampler,
  createComposableTraceIDRatioBasedSampler,
} from '@opentelemetry/sampler-composite';

createCompositeSampler(
    createComposableParentThresholdSampler(createComposableTraceIDRatioBasedSampler(0.3)));

Q: Is it a spec break to use this as the default sampler if a sampling rate is given (whether via OTEL_TRACES_SAMPLER_ARG or otherwise)? See https://opentelemetry.io/docs/specs/otel/configuration/sdk-environment-variables/#general-sdk-configuration and https://opentelemetry.io/docs/specs/otel/trace/sdk/#sampling
I think it is fine for us to use the newer composable samplers for all the OTEL_TRACES_SAMPLER known values.

Q2: Would this be wanted upstream as well? Guessing yes.

See #960 for hooking this up to central config.