Allow publish mode to be a closure

[dwishsan]

Add Closure as possible PublishDir.setMode() input
This is related to #4024

This have been tested and work.

Exemple:

process {
  ext {
    Output_copy = false
  }
  publishDir {
    mode = { task.ext.Output_copy ? 'copy' : 'symlink' }
  }

  withName: 'FOO' {
    ext.Output_copy = true
  }

  withName: 'BAR' {
    ext.Output_copy = false
  }
}

Only results of the FOO process will be copied.

WARNING:
I expect the closure to return a String, so it won't work if it returns null for exemple. Maybe it should handle it?

[pditommaso]

Closing in favour of #4186.