Deprecate observer mode in 5.0 branch

[jamesrb1]

Adds a PurchasesAreCompletedBy enum which is used to set if the RC SDK performs the purchase, formerly referred to as "observer mode". This replaces observer mode and finishTransactions. The change has only been made to public-facing APIs, internal ones will be renamed as part of a subsequent PR.

Other changes:

• Renamed handleObserverModeTransaction(_ PurchaseResult: to recordPurchase(_ PurchaseResult: to try to better communicate what the purpose of the call is. Feedback on if this is needed and what the new name should be is welcome!

• There is a "deprecated" method that doesn't appear to really be a deprecated method, but the deprecation is used as a way of generating a warning where customers are likely mis-using the API. I updated its signature to match the new API. Screenshot of the change for clarity:

  

[fire-at-will]

Looking good! We'll also need to update a few tests in the StoreKit2IntegrationTests, the observer mode section in V5_API_Migration_guide.md, and probably the public documentation as well

[jamesrb1]

Thanks for all your feedback on this @fire-at-will!!!