[ADD] add fnct_search method on related_dispatch_ids field

It allows to search pickings on tree view by dispatch name.
Any operator can be used when searching, not only `in`. I think they should not all translate to the domain `('dispatch_id', 'in', dispatch_ids)`. Some operators such as '<', '>' should raise an error (use a whitelist of allowed operators). `not in` or `=` should directly be used in the domain instead of `in`.