Add custom serializers support

[seagullua]

Right now, the repository has only 2 types of serializers. They both are hardcoded, which makes it impossible to use custom serializers.

I am attaching a proposal on how to add dependency injection support for custom serializers without breaking changes.

I have added System.Text.Json serializer. To enable it you need to add it to SiloBuilder

siloBuilder.AddRedisGrainStorageJsonSerializer();