Snapshot metadata

This isnt for merging yet, just WIP,

Bit of a slow start, but figured while I am still finding my way around the codebase i'll raise this so ppl can peek at the progress more conveniently and give me feedback as i go.
(Btw once this is merged in I'll be raising an issue to add similar tests back into the `SnapshotManifestSpec` after `topic/plan` is merged in)