JSON schema generator - ContentMediaType: base64 needs to be ContentEncoding: base64

Reported by Ketenstandaard.

For xs:base64Binary and xs:hexBinary types we map to JSON schema type string and set the contentMediaType property to base64.

This is not correct and must be the contentEncoding property. See: https://json-schema.org/draft/2020-12/json-schema-validation.html#name-contentencoding