Allow keep to be specified per collector origin

Collector currently provides a global option that controls whether managed worktrees are keep after Collector runs and until what point. It would be useful to be able to control this option per Collector origin. This option would be controlled by the keep key on the worktree key in the collector configuration in antora.yml. For example:

ext:
  collector:
    worktree:
      keep: true
    run:
      command: run-me

The worktree for this particular origin would be preserved indefinitely, regardless of the global option on the extension. Since the worktree is being kept, it would also be unique by origin.