New required and requiredErrorMessage properties for multi fields.

Consider where these properties fit best

Are they still part of https://docs.magnolia-cms.com/product-docs/Templating/Dialog-definition/Field-definition.html#_common_simple_field_properties? They are used in only 1 of 4 complex fields, so they should not be listed under https://docs.magnolia-cms.com/product-docs/Templating/Dialog-definition/Field-definition.html#_common_complex_field_properties. Perhaps adding them as https://docs.magnolia-cms.com/product-docs/Templating/Dialog-definition/Field-definition/List-of-fields/Multi-field.html#_field_specific_properties would be the best solution.