Feat Req: Expose default converters and composite converter

[someth2say]

In this chat we talk about the capability for creating a converter that delegates blocks to another converter.

In Ruby, we have the composite converter that does something exactly that, delegaing the conversion to other converters.
But in java, we have no (native) option for obtaining default converters, even less to the composite converter.

It would be interesting we could obtain proxies to native converters, and ever more interesting being able to create a composite converter (either native or a proxy).

[abelsromero]

We could take inspiration from Asciidoctor.js https://docs.asciidoctor.org/asciidoctor.js/latest/extend/converter/custom-converter/#composition-pattern