feat: replace gson with kotlinx.serialization

[binayshaw7777]

This PR replaces the Gson library with kotlinx.serialization for JSON handling throughout the application.
This change affects network data sources, preference data storage, RSS provider APIs, and security key encoding/decoding.

This also resolves Issue: 994

[JunkFood02]

https://github.com/Kotlin/kotlinx.serialization/blob/master/docs/polymorphism.md

[binayshaw7777]

Made the requested changes. I followed the 1st approach, i.e. going with sealded classes instead of abstract classes. Also marking Serializable to it's subclasses. Now one step more, I also added custom serial name to their subclasses.

[JunkFood02]

I'm unsure if @Serializable is needed on the base class, and I don't think a sealed class is necessary. I propose creating some fake objects and the corresponding JSON produced by previous Gson implementation to compare against in unit tests.

[binayshaw7777]

I understand. I actually followed this rule: https://github.com/Kotlin/kotlinx.serialization/blob/master/docs/polymorphism.md#concrete-properties-in-a-base-class

Moreover, I can you please help me with how do I reproduce this to test it out? I mean the JSON response before and after the implementation. Do we need to write testcases for it? If yes can you help me with the dummy data which is valid for GSON?