Move ObjectSerializer implementation to Azure.Core

[pakrym]

Fixes #13320

Part of Azure/azure-sdk#1578

To quote @tg-msft

We’re introducing a Bring Your Own Serializer model because there a lot of customers who wont come to Track 2 without it. ObjectSerializer is the base abstraction. We’re shipping JsonObjectSerializer : ObjectSerializer to wrap using System.Text.Json. As a customer you’ll still use all the System.Text.Json attributes, naming policies, options, documentation, etc., for your type T. You’ll just tell our client libraries which serializer you’re using via clientOptions.Serializer = new JsonObjectSerializer(new System.Text.Json.JsonSerializerOptions(...)) and we’ll use it for all your custom model types in our clients. You can see more example usage in the tests.

[pakrym]

/check-enforcer override