Add Support for Swift Concurrency

## Summary

Currently, if using the SDK in a project that enables strict concurrency checking, Xcode will complain that the `Amplitude` object is not marked as `Sendable` and is not concurrency safe.

To circumvent these warnings (which may become errors in Swift 6), we can mark the library as `@preconcurrency`

```swift
@preconcurrency import AmplitudeSwift
```

## Motivations

Swift Concurrency is becoming a large part of the Swift language, and moving into 6.0 will be important to add support. This might mean exposing a different kind of object, or migrating the `Amplitude` class itself to an actor.