Add support for metadata semantic on classes

[diegoalexdiaz]

Existing metadata support on properties enables the advertising of what data on a property means, for readers (machine and human-alike) who don't fully understand the layout of its underlying schema. Use-cases relying on semantics involve data-interoperability and data-validation.

The same exact need arises when it comes to classes. Data-readers who can't decipher the semantics behind a class from its schema, need a way to understand them, typically based on a well-known and agreed-upon semantic vocabulary. Terms in such vocabularies not only apply to properties but to concepts overall - usually materialized as classes in a schema.

Thus, the request here is to extend support to capture metadata semantics into classes.

[diegoalexdiaz]

Associated Issue: #811

[javagl]

I added a comment at #811 (comment) which may already address some of the points here. (Maybe there is enough overlap to close this issue, but it may make sense to differentiate some of the "specification-level" aspects from the "technical infrastructure", so I'll leave this open for now)

[lilleyse]

On a related note, we should also add metadata semantic to enums.