feat!: rework extension handling on errors & responses

[obmarg]

Why are we making this change?

GraphQl allows a server to provide arbitrary extensions on responses & errors within a response. Currently cynic only supports decoding these extensions on errors. It does this with generic parameters on the response & error type.

These generic parameters are a little awkward when using the cynic http code - as we need a way to pass through the expected extension types. This led to the addition of a retain_extensions function.

Some users would also like to access extensions on the response itself.

What effects does this change have?

Adds support for extensions on the response type. Rather than adding this as a new generic parameter we now store extensions in a serde_json::Value and provide functionality for deserialising from that. This saves us from adding another generic parameter everywhere, and saves us from having to implement functions like retain_extensions in the http layer.

Fixes #1016

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