For Open Formulieren we need to add custom claim fields for eIDAS company information (open-formulieren/open-forms#4453). The claim for this field is urn:etoegang:1.11:EntityConcernedID:eIDASLegalIdentifier, which is too long according to the ClaimField base_field config

I think it would be best to make the ClaimField base_field configurable, to allow situation specific configuration.

If overriding base_field was done intentionally, we could also make max_length configurable, which is then passed to the base_field config.