In AngularFireFunctions allow for calling `httpsCallableFromURL`

Feature request
========================

In AngularFireFunctions allow for calling `httpsCallableFromURL` (https://firebase.google.com/docs/reference/js/functions.md#httpscallablefromurl), similar to current call call to `httpsCallable`. With Firebase Cloud Functions V2 this is the only way for calling that new functions (since, they are still in preview mode: https://firebase.google.com/docs/functions/beta/callable#call_the_function).

Start at the AngularFireFunctions entry point and compare its existing httpsCallable support with Firebase's httpsCallableFromURL API described in the issue. Done means AngularFireFunctions can call a Cloud Functions V2 endpoint by URL with behavior consistent with the existing callable support; add or update the relevant tests if the implementation identifies them.

Written by the indexing model from the issue text.