Always close BigTable clients

[adutra]

Quarkus tests periodically output the below warning:

*~*~*~ Previous channel ManagedChannelImpl{logId=3, target=localhost:53285} was garbage collected without being shut down! ~*~*~*
    Make sure to call shutdown()/shutdownNow()
java.lang.RuntimeException: ManagedChannel allocation site
        at io.grpc.internal.ManagedChannelOrphanWrapper$ManagedChannelReference.<init>(ManagedChannelOrphanWrapper.java:102)
        at io.grpc.internal.ManagedChannelOrphanWrapper.<init>(ManagedChannelOrphanWrapper.java:60)
        at io.grpc.internal.ManagedChannelOrphanWrapper.<init>(ManagedChannelOrphanWrapper.java:51)
        at io.grpc.internal.ManagedChannelImplBuilder.build(ManagedChannelImplBuilder.java:672)
        at io.grpc.ForwardingChannelBuilder2.build(ForwardingChannelBuilder2.java:260)
        at com.google.api.gax.grpc.InstantiatingGrpcChannelProvider.createSingleChannel(InstantiatingGrpcChannelProvider.java:442)
        at com.google.api.gax.grpc.ChannelPool.<init>(ChannelPool.java:107)
        at com.google.api.gax.grpc.ChannelPool.create(ChannelPool.java:85)
        at com.google.api.gax.grpc.InstantiatingGrpcChannelProvider.createChannel(InstantiatingGrpcChannelProvider.java:243)
        at com.google.api.gax.grpc.InstantiatingGrpcChannelProvider.getTransportChannel(InstantiatingGrpcChannelProvider.java:237)
        at com.google.api.gax.rpc.ClientContext.create(ClientContext.java:226)
        at com.google.cloud.bigtable.admin.v2.stub.EnhancedBigtableTableAdminStub.createEnhanced(EnhancedBigtableTableAdminStub.java:61)
        at com.google.cloud.bigtable.admin.v2.BigtableTableAdminClient.create(BigtableTableAdminClient.java:158)
        at org.projectnessie.versioned.storage.bigtable.BigTableClientsFactory.createTableAdminClient(BigTableClientsFactory.java:151)
        at org.projectnessie.quarkus.providers.storage.BigTableBackendBuilder.buildBackend(BigTableBackendBuilder.java:105)
        at org.projectnessie.quarkus.providers.storage.PersistProvider.produceBackend(PersistProvider.java:81)