This PR introduces the IJsonSerializer interface, with a default implementation of SystemJsonSerializer. Just like #1122, the goal here is threefold:

1. Make it easy for users to control and customize exactly what gets serialized and sent to Shopify's servers, in addition to how the data gets deserialized when it comes back.
2. Support users swapping out System.Text.Json with their favorite Json serializer, such as Newtonsoft.Json, or vice versa.
3. Make unit testing easier and more versatile.
4. Support more Dependency Injection scenarios.

Note: this is currently detached and entirely separate from the existing Newtonsoft serializers/converters we have in the package, and from the static serializers in the ShopifySharp.Infrastructure.Serializer class (though it does use default System.Text.Json serializer settings from there). It's also separate from the System.Json.Text serializers that we have set up in the ShopifySharp.GraphQL namespace.

I want to remedy that quickly and standardize them all under the IJsonSerializer interface.