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).
