Support automatic Identifiable protocol generation in Swift Codegen

It would be awesome if the swift codegen had an option to detect and automatically generate the correct protocols to conform to `Identifiable` when a `GraphQLSelectionSet` has an `id` property.

It is currently possible to write code to conform, but it is fragile:
```swift
// assumes that PeopleQuery.Data.AllPerson.Edge.Node has an id property as specified by .graphql
extension PeopleQuery.Data.AllPerson.Edge.Node: Identifiable {
}
```

## Proposed Solution

The proposed solution would be to add an optional flag and generate the extensions in the current codegen, but it may perhaps be easier to traverse the AST of the generated API as a separate step. Advice is welcome if folks have done this before.

## Reasoning
While it is currently possible to support this solution today- it is tedious to rewrite the same code.
```swift
extension PeopleQuery.Data {
var flattenedEdges: [Self.AllPerson.Edge.Node] {
allPeople?.edges?.compactMap { $0?.node } ?? []
}
}

extension PeopleQuery.Data.AllPerson.Edge.Node: Identifiable {
}

extension PlanetsQuery.Data {
var flattenedEdges: [Self.AllPlanet.Edge.Node] {
allPlanets?.edges?.compactMap { $0?.node } ?? []
}
}

extension PlanetsQuery.Data.AllPlanet.Edge.Node: Identifiable {
}
```

Start by locating the Swift codegen entry point and the current generated API output path; the issue suggests either an optional flag or a separate traversal of the generated API AST. Clarify which GraphQLSelectionSet types qualify and what generated Identifiable conformance should look like, then verify the behavior on selections containing an id property.

Written by the indexing model from the issue text.