Adding Snapshot References Feature

[t-vvidyasaga]

Overview
Adds Snapshot References to the .NET configuration provider for dynamic updates via the cloud portal, with automatic resolution and no code changes.

Key Changes
AzureAppConfigurationProvider.cs

• Detects snapshot references via custom content type.
• Implements ParseSnapshotReference() for JSON parsing.
• Adds Resolve() for snapshot resolution.
• Updates LoadSelected() to auto-resolve references.
• Triggers refreshAll for snapshot references.

SnapshotReference.cs (New)
Model for snapshot reference: name, client, cancellation token.
SnapshotReferenceConstants.cs (New)
Shared constants for content type and JSON fields.

Test Coverage
Includes unit and integration tests for resolution, refresh behavior, and edge cases.

Key Callouts
Resolution: Auto-detected by content type; lexicographic “last wins” for duplicates.
Exceptions:

• Invalid JSON → FormatException
• Invalid composition → InvalidOperationException

Refresh: Always triggers refreshAll, regardless of settings.
JSON Handling: Ignores extra fields, validates snapshot_name, handles empty values.
Edge Cases: Non-existent references and empty names return empty config without exceptions.