Support for `github-token` in `update-discussion` safe output

[mnkiefer]

We want to support workflows as follows:

safe-outputs:
  allowed-github-references: [monalisa/ops-repo]
  create-issue:
    github-token: ${{ secrets.WRITE_TO_OPSREPO_TOKEN }}
    title-prefix: "[Summary]"
    close-older-issues: true
  update-discussion:
    staged: true
    github-token: ${{ secrets.READ_DISCUSSIONS_TOKEN }}
    target: "*"
    target-repo: monalisa/discussions-repo

But currently this is not supported and the compiler fails with the following error:

error: 'safe-outputs/update-discussion' (line 36, col 5): Unknown property: github-token. Valid fields are: allowed-labels, body, footer, labels, max, staged, target, target-repo, title. 'github-token' belongs under 'tools/github', 'on' or 'plugins'