Add support for sharded state bookmarks for substreams (ex. substreams by parent key)

On #20 (closed) and in discussions on !1 (merged), we have uncovered the need to work out additional spec work on complex bookmarking requirements - specifically bookmarks which are tracking distincts "shards" or "subdomains" of the total domain of entities in the stream.

A common scenario, for instance, would be keeping a distinct bookmark for each "project" in the GitLab tap, since virtually all streams must be keyed off of a project_id. For instance, the nested bookmark structure is needed to track the latest replication key for each substream, since each was run at a slightly different time, and each may need to be retried separately from one another if a failure affects one substream and not the others.