feat: add function to update channel in notifications preferences

[stigi]

In this PR

This PR contains two things:

• A new function to easily update a single channel in notification preferences:

user.preferences.update(categorySlug: categorySlug, channelSlug: channelSlug, enabled: enabled) { result in }

• Additions to the Example app to demonstrate that & how it works

Discussion

The /notification_preferences endpoint is funny in the way that a GET request returns a different shape than a PUT request expects. The former returns labels while the later does not expect labels. It doesn't hurt passing them, and the PUT will simply ignore them, but I've opted for making this behaviour explicit in the docstrings, and unit tests.

The solution I went with simply drops the label keys when encoding NotificationPreferencesEntity (see changes in Source/Features/NotificationPreferences/Data/NotificationPreferencesEntity.swift). I tried alternative approaches, but the Harmony framework in use is expecting GET and PUT datasources to have the same entity type, making it impossible to have one with, and one without labels (see this line of code and the Get.T == T, Put.T == T). I've added a comment to the codebase that describes the behaviour.

Test Plan

Tested in the sample App as shown in this video:

notification.preferences.mp4