Validate beta properties as array or singular

The singular vs array aspect of `beta:` properties are currently not being validated, which is creating inconsistencies in the data being published.

` "@container": "@list"` within https://github.com/openactive/ns-beta/blob/master/beta.jsonld indicates whether the property is defined as an array, so a rule in the validator simply needs to be added/updated to take this property into account.