lib/ref: Add OstreeCollectionRef type for globally unique refs

lib/ref: Add OstreeCollectionRef type for globally unique refs

This is a type representing the tuple (collection ID, ref name), which is
guaranteed to be globally unique. It will be used in upcoming commits.

It introduces the concept of a ‘collection’ which is a unique, curated
set of refs which lie in the same trust domain (i.e. all signed by the
same key and validated by the same developer). Flathub might be a
collection, for example; or the set of OS refs coming from a particular
OS vendor.

It includes a function for validating collection IDs.

Signed-off-by: Philip Withnall <withnall@endlessm.com>
Closes: #924
Approved by: cgwalters