Investigate support for SwiftData DataStore for Storage providers

[bauer-andreas]

Problem

SwiftData introduced a new DataStore infrastructure to create a custom data store backend for SwiftData. SpeziAccount currently defines its own StorageProvider interface when storing account details externally.

Solution

We might investigate if we can somehow reuse functionality from SwiftData. For example, we might allow to use SwiftData DataStores to act as a AccountStorage provider.

Additional context

Refer to https://developer.apple.com/wwdc24/10138 for a general overview of the new concepts introduced with iOS 18.

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