The ID of the entity.
For Google-managed entities, the id must be the email address of an existing
group or user.
For external-identity-mapped entities, the id must be a string conforming
to the Identity Source's requirements.
Must be unique within a namespace.
The namespace in which the entity exists.
If not specified, the EntityKey represents a Google-managed entity
such as a Google user or a Google Group.
If specified, the EntityKey represents an external-identity-mapped group.
The namespace must correspond to an identity source created in Admin Console
and must be in the form of identitysources/{identity_source_id}.