Add customizable ignition per worker pool to InfrastructureConfig

[Nuckal777]

Fixes #56

[defo89]

@Nuckal777 could you please add an example to https://github.com/ironcore-dev/gardener-extension-provider-metal/blob/main/example/30-infrastructure.yaml ? maybe with generic ntp?:

  worker:
    worker:
      extraIgnition:
        override: true
        raw: |
          storage:
            files:
            - path: /etc/systemd/timesyncd.conf
              filesystem: root
              mode: 420
              overwrite: true
              contents:
                inline: |
                  [Time]
                  NTP=0.de.pool.ntp.org

And I guess make docs would be good.

[afritzler]

Can you run make docs to reflect the infrastructure api changes in the reference documentation? Apart from that the PR looks good to me.