Feature Request: Allow specifying ownership for trees

[vgchl]

When running rootless containers there's a lot of noise in butane files to set files and directories to have the correct ownership. Having the ability to define the ownership for a tree of files would make creating folder structures like .config/containers/systemd/user/some-service.container etc far more convenient.

I think this could be a convenient alternative to #380 as well.

For example:

trees:
  - path: /home/core/.config
    local: .config
    user:
      name: core   
    group:
      name: core

[travier]

Unless I missed something, this is the same thing as #380. Please comment there.