[storage-image-processing-api] Calling Image Processing API with Cloud Functions for Firebase client SDKs

[gmarizy]

Given the working url:

"https://[my project id here].cloudfunctions.net/ext-image-processing-api-handler/process?operations=${jsonEncode(operations)}"

I didn't succeeded to write the equivalent call with Flutter Firebase Cloud Functions for Firebase client SDK. I tried variation of this snippet with no avail:

FirebaseFunctions.instanceFor()
        .httpsCallable("ext-image-processing-api-handler")
        .call({"operations": jsonEncode(operations)});

What is the way to call Image Processing API from Cloud Functions for Firebase client SDKs ?