Validation is done in a custom hook due to cw'schema limitation: all constraints
must be checked and we need several one in our case (one for each possible
parent type).

A custom vocabulary function is introduced and reused for the 'seda_rule'
relation. There is still some limitation in case where the parent entity has not
yet been created, as we're missing information to go back to the container
holding the scheme definition.

Closes #13562327