Add structured app/device fields to BugsnagSession

[fractalwrench]

Goal

Adds structured app/device fields to BugsnagSession and passes the structured session object into OnSessionCallback to allow users to modify/inspect the payload.

Changeset

• Updated OnSessionCallback to take a BugsnagSession parameter rather than a dictionary
• Updated BugsnagSessionTracker to generate a structured BugsnagSession object
• Renamed session.sessionId to session.id
• Add session.app and session.device
• Make session.user conform to notifier spec with set method + getter
• Add methods for serializing BugsnagSession to JSON
• Add method for deserializing BugsnagSession from a cached payload
• Update [BSG_KSSystemInfo systemInfo] to include memory information by default, to ensure device.totalMemory is populated for sessions
• Updated SessionTrackingApiClient logic to avoid batching of session payloads, as the app/device information can now differ between individual sessions
• Made BugsnagSession and BugsnagUser headers publicly accessible

Tests

• Updated existing test coverage
• Added new tests for serialization/deserialization of sessions
• Verified that a BugsnagSession object is populated during a callback in the example app

[robinmacharg]

Big change set! LGTM.