Add memory caching support in HistoryController

- Updated `EnvelopeGenerator.Extensions.csproj` to include
  `Microsoft.Extensions.Caching.Memory` package.
- Refactored `HistoryController` to use `IMemoryCache` for
  caching functionality.
- Replaced manual dictionary creation in `GetReferenceTypes`
  with a call to `GetEnumAsDictionary<ReferenceType>()`.
- Changed enum type in `GetEnvelopeStatus` to
  `ReferenceType` for consistency.
- Introduced `MemoryCacheExtensions` class with
  `GetEnumAsDictionary<TEnum>` method for efficient enum
  to dictionary conversion.

EnvelopeGenerator.Extensions/MemoryCacheExtensions.cs

@@ -0,0 +1,21 @@
+using Microsoft.Extensions.Caching.Memory;
+
+namespace EnvelopeGenerator.Extensions;
+
+public static class MemoryCacheExtensions
+{
+    public static IDictionary<string, int> GetEnumAsDictionary<TEnum>(this IMemoryCache memoryCache)
+        where TEnum : Enum
+    {
+        var referenceTypes = Enum.GetValues(typeof(TEnum))
+                                 .Cast<TEnum>()
+                                 .ToDictionary(rt =>
+                                 {
+                                     var key = rt.ToString();
+                                     var keyAsCamelCase = char.ToLowerInvariant(key[0]) + key[1..];
+                                     return keyAsCamelCase;
+                                 }, rt => Convert.ToInt32(rt));
+
+        return referenceTypes;
+    }
+}