Provide a mechanism to attach metadata to blocks in jacquard-repo? #14

The motivating usecase here is that PDSes need to be be able to store the tid of the rev when a given block was created in order to be able to provide diffs on request (com.atproto.sync.getRepo with the since parameter).

This clearly needs to be optional, since CAR files have no way of storing this sort of metadata. The best solution I could think of is some kind of AdditionalData associated type on BlockStore that could be passed into get/put functions (and then some kind of interface to attach metatadata to blocks on the MST side?), but if there's better/more ergonomic solution I'd certainly take it..