FR: add the ability to pass abort signal to httpsCallable

[makoffd]

Describe your environment

• Operating System version: macOs Catalina 10.15.4
• Browser version: Google Chrome 81.0.4044.129
• Firebase SDK version: 7.14.3
• Firebase Product: functions

Describe the problem

It's impossible to cancel the httpsCallable call in response to some user event e.g. button click. The proposed solution is to pass down the abort signal to the fetch invocation as described in MDN AbortSignal docs. The signal parameter could be a part of HttpsCallableOptions.

Relevant Code:

interface HttpsCallableOptions {
    timeout?: number;
    signal?: AbortSignal;
}

const controller = new AbortController();
const abortSignal = controller.signal;

functions.httpsCallable('myFirebaseFunction', {
    signal: abortSignal
});

[somq]

Any update on this one?