Add download server config for 3'rd party fw

[GameLord2011]

put an option to change the update download server for 3'rd party fw

[GameLord2011]

my idea is to, instead of compiling the update server with qFlipper do something like this in backend.cpp line 37:
change: m_firmwareUpdateRegistry(new FirmwareUpdateRegistry("https://update.flipperzero.one/firmware/directory.json", this))
to: m_firmwareUpdateRegistry(new FirmwareUpdateRegistry("server.txt".Contents, this))
or something, i don't code w/ the qt framework