Make JsonConvertFactory public sealed

[QuackCola]

Summary

This pr simply makes the class JsonConvertFactory in Utility/Json/IJsonConvert.cs public sealed

Motivation & Context

This change is needed so JsonConvertFactory can be added to custom JsonSerializerOptions. So that you can serialize and deserialize types such as Variant when not using Sandbox.Json.Serialize

Implementation Details

Made the class public sealed so that it can be used externally but not extended.

Checklist

• Code follows existing style and conventions
• No unnecessary formatting or unrelated changes
• Public APIs are documented (if applicable)
• Unit tests added where applicable and all passing
• I’m okay with this PR being rejected or requested to change 🙂

[ElmarTalibzade]

I've noticed that there are a lot of classes in the codebase that could be sealed.

I was considering adding those, but I wasn't sure what's Facepunch's stance is on this and whether they want people to override all of the classes.

[DevulTj]

Sealing stuff we don't want people inheriting is the preferred solution.