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/appsettings.json

@@ -174,6 +174,9 @@
     "Receiver": [],
     "EmailTemplate": [ "TBSIG_EMAIL_TEMPLATE_AFT_UPD" ]
   },
+  "Cache": {
+    "SignatureCacheExpiration": null
+  },
   "MainPageTitle": null,
   "AnnotationParams": {
     "Background": {