Gateway should not work on individual data types, just on JSON.

[DawidNiezgodka]

Consider the scenario in which a user wants to make a query according to the following:

type Query {
getPurchase(id: String): Purchase @topic(name: "purchase", keyArgument: "purchaseId")
}

with a concrete query being:

{
getPurchase(id: "abc")  {
    productId
    }
}

What happens behind the scenes, for example in QueryKeyArgumentFetcher.get():

1. We retrieve the argument ("abc") from the DataFetchingEnvironment, which is JSON representation of the graphql schema,
2. We fetch a value of a specific type (Double, Avro, Protobuf) from the DataFetcherClient,
3. This value is then converted by the framework (which one?) to JSON and delivered to the client.

If we start and end up with JSON, we make unnecessary conversions. Thus, we might refrain from working with different data types (f.e., Double, Protobuf, Avro) in Gateway and work directly with JSON.

For this:
a) MirrorDataFetcherClient has to be rebuilt so that it does not work with TypeResolver<V> but with JSON,
b) MirrorClient should not receive a resolver that works on a given data type but simply on JSON.

Additionally, it might be considered to complete remove the generic V parameter from Gateway.

[DawidNiezgodka]

Changes made in this issue do not involve SubsciptionFetcher and MultiSubscriptionFetcher.