Support appstream licenseref-proprietary SPDX ids and other similar schemes

See @hughsie 's https://www.freedesktop.org/software/appstream/docs/chap-Metadata.html#tag-project_license

> LicenseRef-Proprietary=https://example.com/mylicense.html
>
> A full list of recognized licenses and their identifiers can be found at the SPDX OpenSource License Registry.
>
> Custom licenses which are not in the SPDX registry, like proprietary licenses, can be denoted using the LicenseRef notation. LicenseRef-Proprietary can be used to denote a proprietary license, with an optional URL to the license text following after a = sign.
>
> Although the project_license tag is not mandatory, it is highly recommended to include it.
>
> Examples:
>
> LGPL-3.0+ OR MPL-2.0
> LGPL-3.0+ OR MPL-2.0
> GPL-3.0-or-later
> LicenseRef-proprietary=https://code.visualstudio.com/license

See also https://github.com/search?q="LicenseRef-Proprietary%3D"&type=code

And https://github.com/hughsie/appstream-glib/blob/90993582da90eb9e17601262111c0c3163e7ffb7/libappstream-glib/as-utils.c#L324 and https://github.com/fwupd/fwupd/blob/173d389fa59d8db152a5b9da7cc1171586639c97/src/fu-util-common.c#L1434 and https://github.com/GNOME/gnome-software/blob/0895082fc0b09cf977b18c072c7b3bbb43eff518/src/gs-details-page.c#L2636