Need C++ samples to list and create evaluations

The Workload Manager samples file (google/cloud/workloadmanager/v1/samples) is missing samples for creating or listing evaluations. Add a region tag for listing evaluations so that we can publish the sample in public docs.

Here is a sample for pubsub: google/cloud/pubsub/samples/topic_admin_samples.cc

Start in google/cloud/workloadmanager/v1/samples and compare the existing samples with google/cloud/pubsub/samples/topic_admin_samples.cc. Add samples covering evaluation creation and listing, including a region tag for the listing sample so it can be published in public docs. Confirm the resulting samples are present in the Workload Manager samples file and expose both requested operations.

Written by the indexing model from the issue text.