Today a snapshot repository does not have a well-defined identity. It
can be reregistered with a different cluster under a different name, and
can even be registered with multiple clusters in readonly mode.
This presents problems for cases where we need to refer to a specific
snapshot in a globally-unique fashion. Today we rely on the repository
being registered under the same name on every cluster, but this is not a
safe assumption.
This commit adds a UUID that can be used to uniquely identify a
repository. The UUID is stored in the top-level index blob, represented
by `RepositoryData`, and is also usually copied into the
`RepositoryMetadata` that represents the repository in the cluster
state. The repository UUID is exposed in the get-repositories API; other
more meaningful consumers will be added in due course.
