feature(defer): Partial and Incremental Parsing

[calvincestari]

Closes apollographql/apollo-ios#3277
Closes apollographql/apollo-ios#3145
Closes apollographql/apollo-ios#3147

To recap the progress so far:

• main has some defer changes merged for codegen but they are disabled
• feature/defer-execution-networking is where I'm collecting the rest of the changes that will enable preview releases.

This PR brings parsing of the initial response (partial) and subsequent responses (incremental) to the defer implementation. The changeset includes changes to the templates and rendering but mostly are focused on the execution and networking layers. Details below:

• Removed hasDeferredFragments from the Operation definition. This was only used for the request HTTP headers but that behaviour has changed too and operations now get a deferredFragments property to help with incremental execution.
• Deferrable conformance was moved to the InlineFragment declaration to match the definition of Fragment. This allows us to clean up some of the inline fragment code generation.
• IncrementalJSONResponseParsingInterceptor is a new interceptor built to handle incremental payloads and combining them with previously parsed results.
• There is no collection of deferred fields when a .deferred selection set is encountered because those fields will not be present in the response data, but the deferred fragment is marked for future collection. Incremental execution will then pick up the deferred fragments fields for collection when the relevant response is received.
• GraphQLExecutor now has two entry points for execution; one used for the initial partial response and the other used during incremental execution parsing. They both converge in subsequent execution calls but the difference is in the generic constraints of the function and which selection set they operate on - RootSelectionSet or SelectionSet.
• GraphQLResponse has been broken up into a base struct named AnyGraphQLResponse which provides common logic between GraphQLResponse, used for partial responses, and IncrementalGraphQLResponse which is used for incremental response parsing.
• DataDictMapper was broken out of GraphQLSelectionSetMapper and handles the conversion of JSON data into the DataDict format.
• GraphQLResult now has the ability to merge in data from an IncrementalGraphQLResult. This is used during incremental response parsing and is required because of the way the interceptors require a single 'complete' result to be passed into the call to proceedAsync. The partial and incremental results are built upon each other during each incremental response storing the 'completed' response in the incremental interceptor which is unique to each operation request.
• PathEntry from GraphQLError was broken out into it's own type for reuse in the incremental execution.
• Caching is disabled for now so the cache write interceptor simply ignores caching behaviour for any operation using @defer. This will come in a later PR.

There are no tests included with this PR yet, they're still coming. Last of the tests outstanding is for incremental execution.

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[calvincestari]

@AnthonyMDev I think this PR is finally ready for review.

[AnthonyMDev]

I've just reviewed the production code, and had a few very minor comments. I'm going to review the tests shortly as well!

[AnthonyMDev]

It feels like we could really still just use the one execution function above if we just change it from needed RootSelectionSet to just SelectionSet. It doesn't look like there is any other actual deviation in functionality between these two functions. Am I missing something?

[calvincestari]

OK. I've made a note that we should chat about it this week and if there are changes I'll make a separate PR for that.

[AnthonyMDev]

The tests LGTM! Just a couple minor questions.