Change the declaration of the ID given in the schema declaration

Problem/Motivation

The id property is defined with the following lines.

      id:
      type: string
      label: 'ID'
      nullable: true

Since the ID is a machine name, it should be defined using a more specific type: machine_name. This does not change how the property is stored, but it changes how the property is validated.
Furthermore, it does not need to be nullable.

Proposed resolution

Change the schema for the ID to the following one.

      id:
      type: machine_name
      label: 'ID'