The support for specifying multiple `image_types` for a single compose
has been removed by [1]. This turned out to be problematic, because e.g.
Pungi uses the array type when triggering image builds via osbuild.
Bring back the support for specifying the `image_type` as an array, but
restrict it to a single item. This will cover the Pungi use-case, since
it is always passing a single `image_type` in the array. The array is
then converted to a string in the Hub plugin and passed as such to the
Builder plugin.
Extend unit tests covering the introduced compatibility layer.
[1] c725265081
f21a2de39b