Apollo-ios: Codegen: Add `Identifiable` conformance

[designatednerd]

Great idea from the thread seeking Swift codegen suggestions - anything that has a GraphQLID should conform to Identifiable.

Note: Identifiable is part of the Swift 5.1 release, so it will need to wait until that gets finalized. That's likely well before the updated codegen gets finalized, though.

[rodrigoelp]

Ideally it should auto implement Hashable as well, allowing us to use the objects/structs directly in dictionaries and/or sets.
However, I understand some people might say certain fields should be consider in the hash and some people might vote for adding everything to the hash.

[designatednerd]

@rodrigoelp Hashable and Equatable conformance will be added by default in updated codegen - please see the RFC in #939 for details.

[rodrigoelp]

Ah, bad @designatednerd, I did read that issue (only the part of Equatable). Thanks for the reply and happy new year.