Create an extension that reads common used attributes from a central file

[mmattel]

When we once upgrade to Antora 3

...Reading this correcly, using such an extension in an multi-repo environment, I could make a "central" accessible file (like via rawgithubusercontent) defining common used attributes that can be automatically included without redefining them in each repo again and again?...

Answer - yes

Get asciidoc attributes values from external sources

References #4853