iOS Privacy Pro exit survey

[samsymons]

Task: https://app.asana.com/0/1207619243206445/1207636661001863/f

This PR adds the Privacy Pro exit survey.

It is set up to hide the New Subscriber survey from users who have interacted with the Exit survey, using the exclusionRules feature.

Testing steps:

1. First, set up the app to use this RMF manifest, and check that you get no messages when signed out of Privacy Pro
2. Sign into an active account that has been in use for 14+ days and check that you get the New Subscriber survey - do not interact with it though, since we later want to check that it is hidden when you interact with the Exit survey
3. Override your account to simulate it being in the cancelled state, and check that you get the Exit survey instead - to do this, you can modify RemoteMessagingClient.swift around lines 186-197 to force privacyProIsExpiring to true even if your account is active
4. Open or dismiss the Exit survey, and then check that you do not get the New Subscriber survey the next time RMF refreshes

[github-actions]

Your PR is hosted at https://staticcdn.duckduckgo.com/remotemessaging/config/staging/pre/ios-config.json

[samsymons]

We want to hide the New Subscriber survey from users who had an Exit survey, so we'll use this in exclusionRules on the New Subscriber survey.

[amddg44]

LGTM! Everything behaved exactly as expected 👏