Fetch the pending enrollment requests from the secondary server

[sitaram-kalluri]

Is your feature request related to a problem? Please describe.

When the app restarts, currently there is no way to fetch the pending enrollment requests. Expose a method in at_client which will return all the pending enrollment requests.

Describe the solution you'd like

The enroll verb syntax has "enroll:list" which has its implementation in the at_secondary_server. Add the corresponding methods in at_client to return the server response to the app.

Describe alternatives you've considered

No response

Additional context

No response