Add Sampler.shutdown

[anuraaga]

What are you trying to achieve?

We have TracerProvide.shutdown to allow a top-level entrypoint to shutdown all parts of the tracing SDK. However, we only have shutdown on span processors and exporters, but Sampler, where it is relatively common to have versions that fetch config from a remote source and need to be closed properly, does not. If we want the top-level entrypoint to be able to close all parts of the tracing SDK, Sampler also needs a shutdown method.

Additional context.

Related discussion on the desire for a top-level shutdown: #1288

[Oberon00]

Marked after-ga as per #1288 (comment)