quests: QuestMetrics helpers, README quests section

Services/QuestMetrics.cs

@@ -0,0 +1,16 @@
+namespace RudderSdk.Core;
+
+/// <summary>
+/// Builders for the quest metric strings known to the platform. Purchase
+/// metrics are reported automatically by the shop purchase fan-out; any other
+/// metric is a custom string reported via
+/// <see cref="QuestsService.ReportProgressAsync"/>.
+/// </summary>
+public static class QuestMetrics
+{
+    /// <summary>Metric for purchasing a store offer; auto-reported on purchase.</summary>
+    public static string PurchaseOffer(string offerId) => $"purchase.offer:{offerId}";
+
+    /// <summary>Metric for purchasing a catalog item; auto-reported on purchase.</summary>
+    public static string PurchaseItem(string itemId) => $"purchase.item:{itemId}";
+}