split the namespace out of BackupGroup/Dir api types

split the namespace out of BackupGroup/Dir api types

We decided to go this route because it'll most likely be
safer in the API as we need to explicitly add namespaces
support to the various API endpoints this way.

For example, 'pull' should have 2 namespaces: local and
remote, and the GroupFilter (which would otherwise contain
exactly *one* namespace parameter) needs to be applied for
both sides (to decide what to pull from the remote, and what
to *remove* locally as cleanup).

The *datastore* types still contain the namespace and have a
`.backup_ns()` getter.

Note that the datastore's `Display` implementations are no
longer safe to use as a deserializable string.

Additionally, some datastore based methods now have been
exposed via the BackupGroup/BackupDir types to avoid a
"round trip" in code.

Signed-off-by: Wolfgang Bumiller <w.bumiller@proxmox.com>
Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>