Adding an external extension

If an AsciidoctorJ extension is not already supported in this plugin suite, there is a there is a special extension of type AsciidoctorjGenericExtension.

Groovy

import org.asciidoctor.gradle.model5.jvm.extensions.AsciidoctorjGenericExtension
import org.asciidoctor.gradle.model5.jvm.toolchains.AsciidoctorjToolchain

asciidoc {
  toolchains {
    asciidoctorj.asciidocExtensions {
      myBackend(AsciidoctorjGenericExtension) {
        attributes( foo_color : 'black') (1)
        requires 'foo' (2)
        useGem 'foo', '1.2.3' (3)
        useModule( 'acme.foo:foo', '1.2.3') (4)
        useModule( dependencies.project(':my-backend')) (5)
      }
    }
  }
}

1	Set the specific attributes that this extension might require.
2	If one or more `requires` need to be passed to the engine, define them here.
3	If the extension is only available as a GEM, add the name of the GEM and its version. To use this option, the `org.asciidoctor.jvm.gems` must be applied.
4	If the extension is available as a JAR artifact from a Maven or Ivy repository, provide the Maven module coordinates as well as something that will resolve to a version string.
5	If the extension is provided from the current project hierarchy, it can be referenced as a project dependency.