Add CacheController and caching support for signatures

Introduced a new `CacheController` to manage cached data for
receiver signatures using distributed caching. Added endpoints
to save, retrieve, and delete cached signatures.

Created a `SignatureCacheRequest` model for caching payloads
and a `CacheOptions` class to configure cache settings,
including optional expiration. Updated `Program.cs` to bind
`CacheOptions` to the `Cache` section in `appsettings.json`.

Added a new `Cache` section in `appsettings.json` with a
`SignatureCacheExpiration` property, defaulting to `null`
(no expiration).

EnvelopeGenerator.API/Options/CacheOptions.cs

@@ -0,0 +1,18 @@
+namespace EnvelopeGenerator.API.Options;
+
+/// <summary>
+/// Configuration options for distributed caching.
+/// </summary>
+public sealed class CacheOptions
+{
+    /// <summary>
+    /// Configuration section name in appsettings.json.
+    /// </summary>
+    public const string SectionName = "Cache";
+
+    /// <summary>
+    /// Signature cache expiration time.
+    /// If null, signatures will not expire automatically.
+    /// </summary>
+    public TimeSpan? SignatureCacheExpiration { get; set; }
+}