Add support for PlantUML diagrams in Asciidoc.

What does this MR do?

Allows to include plantuml diagrams inside Asciidoc documents.

Are there points in the code the reviewer needs to double check?

Missing test for lack of experience. Pointers on how to write correct tests are appreciated.

Why was this MR needed?

Including diagrams inside wikis, issues, comments, snippets, etc. is a powerful addition to Gitlab.

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Maybe

What are the relevant issue numbers?

• Issue #17603 (closed)