feature: Emit Identifiable conformance on SelectionSets

[x-sheep]

This is a rework of #548, except this time it will only emit a conformance if the keyFields is set to exactly ["id"] (instead of assuming the presence of an id field is enough).

If a type has a field named id it is actually impossible to override the behavior of Identifiable, so I believe there is no situation in which an automatic conformance to Identifiable isn't desirable.

This PR will not generate a getter named id (as was proposed earlier), so there won't be an unexpected field named id in any case.

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[calvincestari]

I think it would be beneficial to include a test that has id as the key field but it is not included in the selection set; test should not generate Identifiable conformance.

// given
schemaSDL = """
type Query {
  allAnimals: [Animal!]
}
type Animal @typePolicy(keyFields: "id") {
  id: ID!
  species: String!
}
"""

document = """
query TestOperation {
  allAnimals {
    species
  }
}
"""

[calvincestari]

One formatting suggestion.

[x-sheep]

I've added another test.

[calvincestari]

Thanks for following through on this @x-sheep.

[AnthonyMDev]

I'm happy with this approach! Thanks so much @x-sheep!