Base class for the server implementation of the service GkeHubMembershipService.
The GKE Hub MembershipService handles the registration of many Kubernetes
clusters to Google Cloud, represented with the
[Membership][google.cloud.gkehub.v1beta1.Membership] resource.
GKE Hub is currently available in the global region and all regions in
https://cloud.google.com/compute/docs/regions-zones.
**Membership management may be non-trivial:** it is recommended to use one
of the Google-provided client libraries or tools where possible when working
with Membership resources.