Support callback target creation & management

[mwvaughn]

Is your feature request related to a problem? Please describe.
The original CLI supported creation of httpbin callback targets. This was incredibly helpful for debugging notifications, but with the deprecation of agaveapi.co services, it can no longer be relied upon.

Describe the solution you'd like
Add a command namespace tapis callback *. Command could include create, view, and test, and delete. It could be viable to include support for multiple backends.

Describe alternatives you've considered
Manual creation of a callback target is the obvious alternative, but this is a new-ish concept to many users.