add ManagerIdentity field to WorkerDeployment, and API to set/unset (#632)

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<!-- Tell your future self why have you made these changes -->
I want the Worker Controller to stop making changes to a Worker
Deployment if it detects that another client has written to the same
Worker Deployment. This is so that, if a user calls `SetCurrentVersion`
or `SetRampingVersion` from the CLI during an incident, the Controller
does not overwrite that change.

`LastModifiedBy` is not enough for this feature, because we need a way
for the "other client" (likely user CLI in most cases, maybe UI if we
allow such changes through the UI some day) to indicate to the
controller that manual changes are no longer needed and the controller
can safely take ownership over writing to this Worker Deployment again.

I thought about doing this via a `Metadata` on the Deployment, similar
to WorkerDeploymentVersion Metadata, but I can't think of any other use
cases for Deployment-level metadata, and I think this is load-bearing
enough to deserve it's own field, where we can clearly document
semantics.

<!-- Are there any breaking changes on binary or code level? -->
No

<!-- If this breaks the Server, please provide the Server PR to merge
right after this PR was merged. -->
Not implemented in server yet because this is just a proposal

Author: carlydf