[Feature] Add `send()` and `get_dm_room()` to class `User`

[Luna-Klatzer]

Is your feature request related to a problem? Please describe.
Add the ability to directly send a message to a user and allow the Private Room to be fetched by the Client. The only exception being the client itself, since a dm room with itself is forbidden.

Describe the solution you'd like
Adding a new send() method to the User class and implement the functionality to return the dm-room using get_dm_room()

Describe alternatives you've considered
None.

Additional context

• Docs for user rooms