Add new command "getdeviceinfo"

[odin568]

Hi,
there was already a method to get all deviceinfos as a list. When I know the ain, I would like to use the method to get exactly the one device to avoid later filtering.
Beside that, for easier handling, I made FritzBoxSession public - don't know if it was by purpose package private. Makes it easier to quickly try out stuff in my app.
Reach out to me on questions!
BR, odin568

[kaklakariada]

Thank you for your PR and sorry for the late reply!
Please add an entry to the "unreleased" section of the changelog with a description of your change and (if you want) your user name as attribution.

[odin568]

Done.
Thank you!