Validation API for data types (options)

[bertfrees]

This would allow us to

• keep the clients light even if we want to extend the syntax for data type definition in the future (currently limited).
• define data types in Java.

We already have this API in Java (see DatatypeRegistry), but it should also be available through the web API.

See https://github.com/daisy/pipeline-framework/pull/84#issuecomment-145564504.