add values to specify resources on init-containers

Currently, we do not allow users to specify resources for initContainers, we should introduce a value which allows this.

The PodSpecs are defined in [_helpers/pods.tpl](https://github.com/airflow-helm/charts/blob/airflow-8.3.0/charts/airflow/templates/_helpers/pods.tpl#L35-L39).