feat(ol/feature): allow generics for feature.getProperties<T>()

with this change it's possible to pass a generic type `T` when calling `feature.getProperties<T>()` and using with TypeScript.

```
// before: const stop = feature?.getProperties() as Stop
const stop = feature?.getProperties<Stop>()
console.log(typeof stop) // "Stop"
```

closes #14868

src/ol/render/Feature.js

@@ -282,10 +282,12 @@ class RenderFeature {
 
   /**
    * Get the feature properties.
-   * @return {Object<string, *>} Feature properties.
+   * @template {Object<string, *>} T = An optional generic type of the requested feature properties
+   * @return {T} Feature properties.
    * @api
    */
   getProperties() {
+    // @ts-ignore (= this.properties_ as T)
     return this.properties_;
   }