Refactor the ShopifySharp.GraphQL pre-generated classes to use the IJsonSerializer interface where possible

[nozzlegear]

Right now the generated classes in ShopifySharp.GraphQL all have extension methods for serializing and deserializing themselves using System.Text.Json. I'd like to explore whether it's possible to refactor (or disable?) those extension methods to use ShopifySharp's new IJsonSerializer interface, which is built for DI and to let devs choose which serialization library is used with the GraphService (e.g. Newtonsoft, System.Text.Json, etc.).