feat: implement Workspace Update API method

This commit delivers the following:
- Complete the UpdateWorkspace repository method which was previously a stub (GH #848).
    - Apply all mutable spec fields from the update model to the K8s Workspace object and persist via client.Update().
- Add handler tests for successful update, 409 stale revision, and 404 not found.

Not included in this PR and to be addressed in a follow up:
- Retrying on 409 response from k8s server to account for stale cache
    - TODO comment still in code calling out this need

Signed-off-by: Andy Stoneberg <astonebe@redhat.com>

Author: andyatmiami