feat: add values to set init-container resources

[thesuperzapper]

What issues does your PR fix?

• fixes add values to specify resources on init-containers #242
• fixes Resources for init container not specified into helpers/_pods.tpl #808
• closes add values to specify resources on init-containers #684
• closes allow to set resources for init containers #801

What does your PR do?

This PR allows you to set the resource request/limits for the few remaining init-containers for which there was not already values.

This PR adds the following values:

• airflow.initContainers.checkDb.resources (default: {})
  • Sets resources requests/limits for check-db init-containers
• airflow.initContainers.waitForDbMigrations.resources (default: {})
  • Sets resources requests/limits for wait-for-db-migrations init-containers
• airflow.initContainers.installPipPackages.resources (default: {})
  • Sets resources requests/limits for install-pip-packages init-containers

Checklist

For all Pull Requests

• Commits are signed off
• Commits have semantic messages
• Documentation updated
• Passes ct linting