update/workspace.v1: Add TagAssignment entity to Workspace API

- Add TagAssignments as a first-class EntityType
- Update docs to use the tag int enum values in tag resource
  entity_ids rather than stringified versions of the enum values

[GENERATED_COMMIT]

Change-Id: Ibf666322970556b2d23e32c87a0613201ffdf7f4

Author: Arista Jenkins