Support user artifact path for CVD creation with canonical config

[k311093]

Add support for the notation @user_artifact to specify user artifact path at canonical config.

Example config for using user artifact while launching CVD with canonical config.

{
  "env_config": {
    "common": {
      "host_package": "@user_artifact/$user_artifact_id"
    },
    "instances": [
      {
        "vm": {
          "memory_mb": 8192,
          "setupwizard_mode": "OPTIONAL",
          "cpus": 8
        },
        "disk": {
          "default_build": "@user_artifact/$user_artifact_id"
        }
      }
    ]
  }
}