This fixes an issue on shutdown where threads could copying the shared_ptr\<TracerProvider> at the same time it is being set to `nullptr`, leading to a data race and invalid access (see the test in src/mongo/otel/traces/tracer_provider_service_test.cpp). Using a thread_local VersionedValue should be very efficient for callers of this code (e.g. starting spans).

While the `thread_local` imposes some constraints on how the `TracerProviderService` can be used, in practice we are only calling it from `span.cpp` so it is easy to ensure the constraints are met.