Ability to set custom path for DownloadFile & InputFileGenerated

@Nullable String path in DownloadFile would allow implementing features such as:

• Download folder setting (without need in restarting & moving all already downloaded files to a new directory manually by client)
• Client would be able to control directories' names
• Save As...

Probably more.

@Nullable String path in InputFileGenerated would, for example, allow saving generated media directly to the gallery.

Currently clients have two options: copying generated file after generation finish (which would double disk space usage per file) & moving generated file after generation finish (which probably? would waste data usage, if user tries to upload same generated file again later).