Support !includesub in PlantUML preprocessing

[carun]

From https://antora.zulipchat.com/#narrow/stream/282400-users/topic/Antora.20Kroki.20Includes/near/379417451

[ixchelruiz]

Thank you @carun for opening the issue. I just wanted to add a different view on the same use case!

Issue: The rendering of the diagram is not as expected!

vs

Assumption:

A) Maybe we need to use something antora compatible as the reference path, but could not yet figure out what that might be (the sample contains one other variant).

@startuml !$core_path=partial$uml/core/core.puml : :
https://github.com/madmas/demo-antora-doc-A/blob/main/modules/ROOT/partials/uml/domain/refdia-antorapath.puml
or
@startuml !$core_path=../core/core.puml : :
https://github.com/madmas/demo-antora-doc-A/blob/main/modules/ROOT/partials/uml/domain/refdia.puml

B) Maybe I do need to include pump files to antora preprocessing?

C) Includes will not work at all ( here the purpose is to leverage the configuration of the layout)

This is the sample project to demonstrate the problem: https://github.com/madmas/demo-antora-doc-A
Rendered Antora page can be found here: https://madmas.github.io/demo-antora-doc-A/Doc-A/index.html
In my diagram, I need to reference elements from another PlantUML file as shown in https://github.com/madmas/demo-antora-doc-A/blob/main/modules/ROOT/partials/uml/domain/refdia.puml

[ggrossetie]

I've updated the issue title since the extension will need to implement the !includesub directive and also evaluate PlantUML attribute definition (i.e., !$core_path=partial$uml/core/core.puml).

At some point, it might be worth to extract this logic into a dedicated library that will preprocess a PlantUML file (given a context) into a standalone PlantUML file (i.e., it will resolve all external resources).