[native_assets_cli] JSON nest config extensions

[dcharkes]

Currently we could have conflicts between us as protocol authors wanting to add an extension to the config and SDKs/asset-type authors wanting to add an extension.

@mkustermann suggested we should nest the config extensions:

{
  "config" : {
    "extensions" : {
      "code" : {
        "target_architecture": "x64"
      }
    }
  }
}