cubicweb #14411726 provide migration tools for constraint [open]

with recent releases of CW (> 3.21), most constraint handling is now done at the database level. This is fine but we should also provide helper functions for some case of migration...

For instance, if one want to change some static vocabulary, it currently has to compute the database constraint to drop it before migrating existing value. This is hard to do properly and error prone, so CW should provides official helpers for such task (similarly to the constraint_removed context manager)

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