Add support for xsd:restriction

[henkesn]

Creating a schema from WSDL/XSD which have xsd:restriction raised an error. Would be good to have that fixed by simply ignoring the restrictions in a first step. Some cases perhaps could also be mapped to enum later.

Example:

<xs:simpleType>
  <xs:restriction base="xs:string">
   <xs:enumeration value="optional"/>
   <xs:enumeration value="required"/>
   <xs:enumeration value="prohibited"/>
  </xs:restriction>
</xs:simpleType>

[pso-aeb]

XSD-Doc: https://www.w3schools.com/xml/schema_facets.asp
Should use the base as type for the field.
Map to enum seems plausible.
All other possible restrictions don't have a proper correspondence in GraphQL (AFAIS). But we could put them in the comment for the field.