[v3] In Memory Mode

[kkostov]

This PR formalises in-memory mode of operation for the SDK. In this mode, none of our default processors will use client side persistence to disk.

try await TelemetryDeck.initialize(
    appID: "<YOUR-APP-ID>",
    namespace: "<YOUR-NAMESPACE>",
    inMemoryOnly: true
)

Features unavailable in in-memory only mode:

• Persistent event caching: events that haven't been sent will be discarded when the app terminates.
• New-install detection
• Session retention metrics
• Cross-launch duration persistence: duration events are usable only during the lifetime of the app.