Firebase Sync Feature Added

[rappc87]

### How to Set Up Firebase Sync ###

Step 1: Create a Firebase Project
Go to the Firebase Console.
Click "Create a project".
Enter a name (e.g., CloudStream Sync).
Disable Google Analytics (optional).
Click "Create project".

Step 2: Enable Firestore Database
In the sidebar, go to Build > Firestore Database.
Click "Create database".
Choose a Location (e.g., eur3 or us-central).
Select "Start in test mode".
Click "Create".

Step 3: Get Your Credentials (Android Method)
Click the Gear icon (Settings) > Project settings.
Scroll to "Your apps" and click the Android icon (little robot).
Package name: Enter com.lagradost.cloudstream3.
Click Register app.
Click Download google-services.json.
Open the downloaded JSON file with a text editor (Notepad, VS Code) and find these lines:
"project_id" -> Your Project ID
"mobilesdk_app_id" -> Your App ID
"current_key" -> Your API Key

Step 4: Configure CloudStream
Open CloudStream.
Go to Settings > Accounts & Security > Firebase Sync.
Enter the 3 values you found in the JSON file.
Click Connect & Sync.

[phisher98]

At the moment, when the selection is changed on one device, it automatically updates on the other device as well. While the sync speed has improved, I don’t think this behavior is desirable.

Could we exclude the selected extension from syncing? Personally, I’m not a fan of this approach, as users who share credentials for sync may not expect selections on one device to affect another.

[fire-light42]

This is a very good feature that has been both requested a lot, and tried both in extensions and other pull requests.

However you still have some core architectural problems that I want to be fixed before I can do a full indepth review.
These are "Security" and what you call "Zombie keys".

1. Security, while CloudStream has a few security problems, storing the entire backup on the cloud without any sort of password/auth is really bad.
   The very first thing they write in the "Firebase" setup is to never ever use "test mode" in production. So please fix your firebase rules, as otherwise anyone with the information will be able to download and modify your entire backup, which in this case even allows for Zero-click RCE with loading custom plugins/extensions. Storing plugins in a "sync" feature is a bit weird and dangerous. If you really want plugins to be shared across devices, then only sync their corresponding urls and prompt the user for a popup detailing all plugins and repositories before installing any of them.

Moreover, I would also want some sort of multi-user database here, as it is quite frustrating for each user to create a separate "Firestore Database". For example
a few users should be able to share the same database, like family members. This is not strictly needed, but I expect something like this will come for free when you make the firebase rules more strict, like https://firebase.google.com/docs/rules/basics#content-owner_only_access.

2. Zombie keys. I have already written a bit about it here if you want some extra background: #1641 (comment)
   However, the core issue is that keys will have conflicts and may be deleted. As you noticed, if we remove a key how should that be updated to other devices because the current system is a merge only. Your solution right now only works on resume watching and plugins by adding a RESULT_RESUME_WATCHING_DELETED, and addedDate+isDeleted. This does not scale well, and requires all keys to either embed additional metadata, or store some extra data making it ugly and harder to maintain. Unfortunately, everything must have this to prevent very weird sync bugs, so you can't get away with only adding it on the essential data types. A more generic system that works on all keys reliably is needed to avoid this coupling of sync and the rest of the codebase.

This is only the most critical aspects of this pull request, and expect more comments about:

1. Bugs/crashes
2. Performance
3. Localization
4. Code quality
5. Dependencies
6. UI+UX and navigation

After you have finished these two major hurdles.