Extend Gerrit's Comment Markdown syntax

It has been suggested that the CI Failure Analysis bot should post longer context log snippets where relevant, but this would lead to possibly undue visual noise and overly long comments.

The ideal way to work around this would be to use collapsible elements in comments, but gerrit's built-in markdown syntax is rather limited and does not support any such tags like <details></details>.

A cursory review of the Gerrit plugin development guide does not show any relevant integration points, so a deeper look at implementation difficulty is needed.