Make cluster aware versions of @cpattr and @mvattr

[cary]

These should, naturally, when the either of the objects involved are cluster members, behave accordingly (e.g. not duplicate attributes across cluster members; trigger the threshold action; etc).

They should gracefully handle mixed cases (e.g. 'from object' not a cluster member, but 'to object' part of a cluster) appropriately to the point of behaving like their non-cluster versions if neither object involved is a cluster member.

I'd say just make @cp/mvattr cluster away, but I can see situations where you would want to ignore cluster membership coming up too easily.

[Ashen-Shugar]

Looking at how clusters work internally, it might be easier to set up a @cluster/clone to clone an existing cluster than to cpattr it over.

There's too many considerations to take into account for a 1 to 1 copy of attributes. Thresholds just being a small one.

For example, what happens if the target cluster contains said attribute, do you copy over it? Keep the data? If the attribute is locked? Permissions on the attributes?