WithDisplayBuilderInterface follow-ups

Also, if we try to improve our config schema...

From:

    display_builder:
      type: string
      label: "Used Display Builder"

To something like that:

    display_builder:
      type: string
      label: "Used Display Builder"
      constraints:
        ConfigExists:
          prefix: 'display_builder.display_builder.'

From:

    sources:
      type: ignore
      label: "Sources"

To something like that:

    sources:
      type: sequence
      label: 'Sources'
      sequence:
        type: ui_patterns_source.[id]
        label: 'Source'

extending DefaultLazyPluginCollection

Is it unlocking new cool Drupal feature like automatic casting ? automatic dependency calculation ? so we can reduce the complexity of our code