cubicweb-container #3524223 [clone] allow original and cloned containers to be different etypes [validation pending]

C1, C2 being two container etypes with overlapping relations, we want the clone adapter to handle cloning in either direction and effectively clone the right subset of rtypes and etypes.

This is already done in one client cube and maybe can be retrofitted into container without ugly consequences.

done in2.7.0
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