Move SuggestionService to WordPressKit

[guarani]

Expected

SuggestionService should probably live in https://github.com/wordpress-mobile/WordPressKit-iOS, since that's where all (?) the API calling code lives.

This also applies to the soon-to-be-added XpostSuggestionService.

Actual

SuggestionService is currently in WordPress-iOS and isn't in WordPressKit.

[guarani]

A potential downside to this could be that we have to create a separate model for UserSuggestion and SiteSuggestion. At the moment, the same SiteSuggestion type that is a NSManagedObject also conforms to codable, so it serves as both a Core Data entity and a model to deserialize the network response objects into.
If we move these types to WordPressKit, I think it would mean creating another model there, although I'm not 100% certain that it's required. Will need to do some more investigation.

[dangermattic]

Thanks for reporting! 👍

[jkmassel]

WordPressKit is being replaced by wprs, so I'm closing this one.