Support attribute references in attribute values in antora-assembler.yml

Currently Assembler does not resolve attribute references in attributes defined in antora-assember.yml file, e.g.

asciidoc:
  attributes:
    source-highlighter: rouge
    rouge-css: style
    toc: left
    sectnums: ''
    test: 'The year is {year}'

The test attribute is resolved to 'The year is {year}' even if you provide the attribute e.g. npx antora --attribute year=2029 antora-playbook.yml. In order to solve the issue the code already used in the antora repository needs to be integrated in the Assembler repository.