New Get Namespace Limits API

[Brendan Doyle]

Hi everyone,

I've added a new get API to the controller that will return the limits set
for a namespace (i.e. invocationsPerMinute, concurrentInvocations,
firesPerMinute). It will return the system default if limits have not been
set in the db for that authorized user. You can see the PR here:

https://github.com/apache/openwhisk/pull/4899

Thanks,
Brendan D.