AsyncClient download, thumbnail... make timeout optional

[cy8aer]

It would be nice to have an optional timeout setting for AsyncClient download, thumbnail... As I understand it is expected that the end user should cancel a download. If the client itself needs e.g. to download icons of images/thumbnails there should be an optional timeout setting so that such systems do not hang when e.g. offline.

Something like

timeout: Optional[int] = 0

in the function's definitions (and using it in self._send calls) should do the work.

[cy8aer]

Here it is really needed: https://gitlab.com/cy8aer/bluepill/-/issues/28