Feature request: Ability to specify ConfigMap instead of startupScriptContent for init scripts

There are some drawbacks to using the Value startupScriptContent as the initialization script. For example, if the localstack helm chart is a subchart, rendering startupScriptContent doesn't have access to Values or Files from the parent chart. In this case, it's necessary to mount the initialization script manually, for example:

localstack:
  enableStartupScripts: false  # We mount the startup script manually below
  volumeMounts:
    - name: localstack-init-scripts-config
      mountPath: /docker-entrypoint-initaws.d
  volumes:
    - name: localstack-init-scripts-config
      configMap:
        name: my-custom-localstack-init-scripts

It would be handy if we could mount a custom ConfigMap into localstack's /etc/localstack/init/ready.d directory. For example:

  enableStartupScripts: true
  startupScriptConfigMap: my-custom-localstack-init-scripts

would mount the given ConfigMap to the init script entry point.

Start by inspecting the LocalStack chart templates and values handling for startup scripts, then render the chart with a custom startupScriptConfigMap value. Done means the rendered manifests mount that ConfigMap at /etc/localstack/init/ready.d while preserving the existing startup-script behavior.