Fix GRPC channel leak

[shunping]

Description

Problem

When running Portable/YAML pipelines, gRPC outputs severe error logs indicating that ManagedChannel instances were garbage collected without being shut down:

SEVERE: *~*~*~ Previous channel ManagedChannelImpl{logId=..., target=...} was garbage collected without being shut down! ~*~*~*
    Make sure to call shutdown()/shutdownNow()
java.lang.Throwable: java.lang.RuntimeException: ManagedChannel allocation site
    at org.apache.beam.vendor.grpc.v1p69p0.io.grpc.internal.ManagedChannelOrphanWrapper$ManagedChannelReference.<init>(...)

Cause

Three places in the Java Fn Harness allocated gRPC ManagedChannel instances without shutting them down upon task completion or harness shutdown:

• ExternalWorkerService: A ManagedChannel was created to query the provision service for worker options, but was left unclosed after obtaining provisionInfo.
• BeamFnDataGrpcClient & BeamFnDataGrpcMultiplexer: BeamFnDataGrpcClient created a ManagedChannel per BeamFnDataGrpcMultiplexer, but neither stored nor shut down the channel when close() was called.
• FnHarness: The control service ManagedChannel was created during harness startup but was never shut down when FnHarness.main() finished.

[Abacn]

Thanks. I was hopping the leak could have resolved in #39390, where BeamFnDataGrpcMultiplexer is closed there. Turns out there are still missing piece here.