Implement _get, _put, _delete, PrivateHiddenKey

[JeremyTubongbanua]

Is your feature request related to a problem? Please describe.
Implement AtClientImpl.get(PrivateHiddenKey) just like the rest of the AtKey types.

• _get(PrivateHiddenKey)
• _put(PrivateHiddenKey)
• _delete(PrivateHiddenKey)
• Implement new functionalities into the REPL