[NT-780] Add device identifier to prepared request headers

[justinswart]

📲 What

Adds the device identifier to our prepared request headers for requests to v1 endpoints and GraphQL queries/mutations.

🤔 Why

In order to properly attribute our Completed Checkout server-side events to Optimizely experimental variants, we need to send the device_distinct_id to the server so that Optimizely tracking calls can be made using the device_distinct_id instead of the user's ID.

🛠 How

Added a header, Kickstarter-iOS-App-UUID which will be set using the device's identifierForVendor value.

Note: As mentioned in #1044, according to the Apple docs, the identifierForVendor can be nil but this is only when the device has been restarted and not yet unlocked. There should not be any scenario in which we're instantiating our Service struct when this value is nil but if that were to happen then this value would just coalesce to a random UUID string.

✅ Acceptance criteria

Run the app in the simulator and using Charles Proxy observe the following:

• The Kickstarter-iOS-App-UUID header is present in all requests to the v1 API and its value is the device's identifier.
• The Kickstarter-iOS-App-UUID header is present in all requests to GraphQL and its value is the device's identifier.
• Deleting and reinstalling the app generates a new value.

[justinswart]

In order to not make tons of changes I haven't alphabetized these properties for now. I have plans to do this when we do some refactoring of this type soon.

[ifbarrera]

lgtm! 👏