Centralize cache key definitions in CacheKey class

Refactored cache key usage by introducing a new static CacheKey class in the Application.Common namespace. Replaced the private DefaultConfigCacheId in ConfigService with CacheKey.DefaultConfig. Updated using statements accordingly. This change improves maintainability by centralizing cache key management and sets the stage for future cache key consolidation.

EnvelopeGenerator.Application/Common/CacheKey.cs

@@ -0,0 +1,13 @@
+namespace EnvelopeGenerator.Application.Common;
+
+// TODO: merge other cache keys here as well, e.g. for templates, etc.
+/// <summary>
+/// 
+/// </summary>
+public static class CacheKey
+{
+    /// <summary>
+    /// 
+    /// </summary>
+    public static readonly Guid DefaultConfig = Guid.NewGuid();
+}