feat: Allow additional custom properties on service descriptors

- Added support for custom properties in the IJWK interface to enhance interoperability with services that extend the base definition.

Author: brianorwhatever

src/lib/interfaces.ts

@@ -90,6 +90,12 @@ export interface IJWK {
 
     /** The types of messages the service accepts */
     accepts?: string[]
+    /**
+     * Allow additional custom properties on service descriptors. This is required
+     * for interoperability with services whose specification extends the base
+     * definition with extra fields not (yet) known to this library.
+     */
+    [propName: string]: unknown;
   }
 
   /**