feat: expose apisix config for standalone mode support

[ssc806]

This PR adds configuration options to support the API-driven standalone deployment mode for the APISIX Ingress Controller.

Motivation

When deploying APISIX and the Ingress Controller in separate pods (but without etcd), the Ingress Controller needs to know the address and credentials of the APISIX Admin API to push configurations. The current chart does not expose a way to configure the apisix block in the controller's config.yaml or inject environment variables into the adc sidecar container (which handles the communication in standalone mode).

Changes

1. ConfigMap: Added support for rendering an apisix block in config.yaml from .Values.config.apisix. This allows setting base_url and admin_key.
2. Deployment: Added support for iterating over .Values.deployment.adcContainer.env to inject custom environment variables (like APISIX_BASE_URL and APISIX_API_KEY) into the adc-server container.

Example Usage (values.yaml)

config:
  provider:
    type: apisix-standalone
  apisix:
    base_url: http://apisix-admin:9180/apisix/admin
    admin_key: edd1c9f034335f136f87ad84b625c8f1

deployment:
  adcContainer:
    env:
      - name: APISIX_BASE_URL
        value: http://apisix-admin:9180/apisix/admin
      - name: APISIX_API_KEY
        value: edd1c9f034335f136f87ad84b625c8f1