Allow adding extra metadata with makepkg

It would be nice if it was possible to add extra metadata to the .PKGINFO generated by makepkg.

This feature request is motivated by the interoperability with PL ecosystems like Haskell: For example, a tools like arch-hs needs a mapping between ArchLinux package names and the "native" Haskell package names located on Hackage. Currently, there is the convention to prefix Haskell packages with haskell- (See https://wiki.archlinux.org/title/Haskell_package_guidelines), but some packages do not fit nicely into that scheme and require manual mapping of the names (Found here). Since it can be a bit tedious to keep such a mapping in sync with the one used by other packagers I'd like to store the corresponding Haskell name in an xdata field in the .PKGINFO shipped with the ArchLinux packages, and use that when I need to check whether a particular Haskell package I wish to depend on is already in our extra repositories.

The extra metadata could be set in PKGBUILD field, for example:

metadata=(
    key1=value1
    key2=value2
)

That metadata is stored as xdata entries like

xdata = key1=value1
xdata = key2=value2