Add support for bridging to Objective-C

The goal is to provide the following:

- Native Objective-C implementation of QuerySet since using generics in Swift means we can't easily use the Swift implementation from Objective-C
- Release a Pod spec with Objective-C implementation for using in current Objective-C projects without Swift
- Methods to retrieve Objective-C objects from Swift
- Methods to Swift objects from Objective-C

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- [x] Implement `Attribute` bridging.
- [x] Implement `QuerySet` bridging.