Add OfferCall method (by @gamersindo1223)

[fourjr]

Saw this in #560 and thought that it might be useful to add in the main repo

[tulir]

Doesn't seem particularly useful

[fourjr]

Okay, understand your perspective

[tulir]

Note that Client.DangerousInternals() exists so you can do whatever you want outside the library too (it doesn't require forking).

[fourjr]

I tried using that method and unfortunately Client.encryptMessageForDevices is not exposed, only Client.encryptMessageForDevice. Is there any way you can add this to the DangerousInternals @tulir?