New ontapi and Jena assembers.

[afs]

Issue created from a comment on #3152

@afs

@sszuev - jena-ontapi question - does it wire itself in the assemblers for the old API?

@sszuev

"new" model doesn't work with Assembler and FileManger classes directly, it contains its own interface GraphRepository, which is open and can be extended.
Having the specification, I can extend document management in the "new" model, now this management is very simple, but it seems to me sufficient in most cases.

[afs]

@sszuev --

Jena assemblers are a way to write and RDF description that is used to build java objects. For example, they are used in Fuseki to create OntModels that can be used in dataservices.

The assembler associated with old API is [OntModelAssembler](https://github.com/apache/jena/blob/main/jena-core/src/main/java/org/apache/jena/assembler/assemblers/OntModelAssembler.java

I think this is the last piece missing before retiring the old "ontology" apckage in jena-core in favout of the jena-ontapi.

• if there is an alternative to old OntModelAssembler.

Only the top level OntModelAssembler needs a replacement. Old old "ontology has two other related assembers for sub-components:

• OntModelSpecAssembler
• DocumentManagerAssembler

It does not have to be compatible - it has to replace common functionality. It is also a chance to simplify.