main/postmarketos-base-ui: split usb-tethering into subpackage (MR 4788)

this allows to opt-out of usb-tethering on pmOS devices with
`apk add !postmarketos-base-ui-networkmanager-usb-tethering`
this is useful for developers with laptops on pmOS / dailydriving
those laptops on pmOS.

[ci:skip-build]: Already built successfully in CI.