[ENH] Add metalayers for EmbedStore, DictStore and TreeStore frames #546
Description
In order to detect whether a file (or frame in general) contains any of EmbedStore, DictStore and TreeStore, it would be good to add a new metalayer of the header of the frame containing these different objects. It is important to add a metalayer and not a vlmetalayer, as the former are store in the header of blosc2 frames, so access is faster.
Incidentally, EmbedStore already has a "b2meta" metalayer in-place, but I don't see the same in DictStore and TreeStore (probably an oversight). The metalayers for DictStore and TreeStore can be something like: {"b2dict": {"version": 1}} and {"b2tree": {"version": 1}} respectively.
Once all these metalayers are in place, one can extend the work in #451 to guess the type of the store for using it with the blosc2.open() machinery.