Enhancements to `TeachableAgent.learn_from_user_feedback`

[qingyun-wu]

          @rickyloynd-microsoft, minor comments that could be addressed in a different PR.

• learn_from_user_feedback can be a long running job-- makes many API calls. Some notion of logging/progress bar would be helpful
• learn_from_user_feedback could return/yield diff in learning from that call

What do you think?

Originally posted by @gagb in #278 (comment)