Enforce namespace unique constraints

This is the update to actually enable namespace support.

At the moment, I pass a namespace as a query parameter in order to support namespace scoping on the v1beta urls.  Once v1beta3 lands, I will modify to pass namespace in the path.  Note the client libraries were modified to make this easy to do, so should be a trivial change once ready.

I broke the commits into logical sections to improve readability.
The thing I want is for this to not take an api.Context. There's no reason not to take a string, is there?

(also, all other client methods shouldn't take api.Context, but I won't block this PR on that.)