Allow any field type inside multi fields

I stumbled upon this small issue that isn't allowing me to add arbitrarily typed fields to a multi-field using the DSL. I can add them at the moment by using:

    new MultiFieldDefinition("fieldName") as ( /* field definitions with arbitrary types */ )

This PR should fix this so you can define:

    "fieldName" multi ( /* field definitions with arbitrary types */ )

At the moment the syntax above will only work if putting just string fields in the multi.