Also update the Docs and fixup the HTTP API to return proper errors when someone attempts to use Namespaces with an OSS agent.
Add Namespace HTTP API docs
Make all API endpoints disallow unknown fields
This has an explcit unit test already which somehow passes at least some of the time. I suspect it passes because under some conditions the actual KV delete fails and returns non-zero as well as printing the warning which is what is being checked for in the test.
For some reason despite working for quite some time like this, I now have a branch in which this test fails consistently. It may be a timing/env issue where another process running an agent causes the delete to be successful so the command returns a 0 by chance. Either way this is clearly wrong and fixing it stops the test being flaky in my branch.
