This is a preliminary to elastic#49060.

It does not introduce any substantial behavior change to how the blob store repository
operates. What it does is to add all the infrastructure changes around passing the cluster service
to the blob store, associated test changes and a best effort approach to tracking the latest repository
generation on all nodes from cluster state updates. This brings a slight improvement to the consistency
by which non-master nodes (or master directly after a failover) will be able to determine the latest
repository generation. It does not however do any tricky checks for the situation after a repository operation
(create, delete or cleanup) that could theoretically be used to get even greater accuracy to keep this change
simple.
This change does not in any way alter the behavior of the blobstore repository other than adding a better "guess"
for the value of the latest repo generation and is mainly intended to isolate the actual logical change to how the
repository operates in elastic#49060