[Bug] Firebase Functions - GetHttpsCallable actually sends a POST request, should be renamed

### [REQUIRED] Please fill in the following fields:

* Unity editor version: 2021.1.17f1
* Firebase Unity SDK version: 8.8.0
* Source you installed the SDK: .unitypackage
* Problematic Firebase Component: Functions
* Other Firebase Components in use: Auth, Firestore
* Additional SDKs you are using: N/A
* Platform you are using the Unity editor on: Windows
* Platform you are targeting: Android
* Scripting Runtime: IL2CPP

### [REQUIRED] Please describe the issue here:

I am using Firebase Functions with Express routing set up, e.g. in my index.ts file (external to the Unity project):

`api.get("/products", helloWorld);`

When I call this route within my Unity project, using GetHttpsCallable in a C# file:

`var func = _firebaseFunc.GetHttpsCallable("api/products");`

It turns out a POST request is sent by default (using SmartSniff)

From reading about this (StackOverflow [1](https://stackoverflow.com/questions/65039728/with-flutterfire-cloud-functions-are-all-requests-post), [2](https://stackoverflow.com/questions/57266680/how-do-i-do-a-get-for-firebase-functions-httpscallable)) it seems this is intended behaviour - Firebase cloud functions indeed only accept POST requests: https://firebase.google.com/docs/functions/callable-reference

However, it would certainly solve a lot of headaches if the SDK method call did not begin with the word 'GET'. The only error message I was able to get was 'INTERNAL' in the Unity editor, or 'Not Valid JSON' when building to device, and as a result took some time to debug.

After creating a duplicate route with 'post' alongside my 'get' route, the call began to work as expected:
```
api.get("/products", helloWorld);
api.post("/products", helloWorld);
```

#### Steps to reproduce:

As above

#### Relevant Code:

As above