🚀 Feature: Allow passing in custom fetch imp

### 🔖 Feature description

In the constructor of the client, it would be nice for users to be able to provide their own fetch implementations.

### 🎤 Pitch

Why?

Because frameworks like SvelteKit, allow prefetching data on the server and exposing the promise to the client. For this to work and to avoid re-fetching twice (on the server and the client), the appwrite SDK client needs to be able to receive the custom sveltekit fetch implementation.
https://svelte.dev/docs/kit/load#Making-fetch-requests

### 👀 Have you spent some time to check if this issue has been raised before?

- [x] I checked and didn't find similar issue

### 🏢 Have you read the Code of Conduct?

- [x] I have read the [Code of Conduct](https://github.com/appwrite/appwrite/blob/HEAD/CODE_OF_CONDUCT.md)