feat: Add support for custom Keycloak providers and themes via init containers

[llulioscesar]

Summary

• Add extraInitContainers support to download custom JARs (providers, themes, SPIs) at startup
• Add extraVolumes and extraVolumeMounts for mounting custom files to Keycloak
• All new options are optional and backwards compatible

Changes

• templates/keycloak-deployment.yaml: Add support for extraInitContainers, extraVolumes, extraVolumeMounts
• values.yaml: Add new options with usage examples
• values.schema.json: Add schema validation for new options
• Chart.yaml: Bump version to 1.3.1

Usage Example

keycloak:
  extraInitContainers:
    - name: download-providers
      image: busybox
      command:
        - sh
        - -c
        - |
          wget -O /providers/my-theme.jar https://example.com/my-theme.jar
      volumeMounts:
        - name: providers
          mountPath: /providers

  extraVolumes:
    - name: providers
      emptyDir: {}

  extraVolumeMounts:
    - name: providers
      mountPath: /opt/keycloak/providers