Add ability to manually clear cache

### Feature

Currently there's no (easy) way to allow users to clear the cache in Editor or at runtime. However, it can be done from C++ code.

The closest thing we have in the UI is the[ `maxItems` property on `CesiumRuntimeSettings`](https://cesium.com/learn/cesium-unity/ref-doc/classCesiumForUnity_1_1CesiumRuntimeSettings.html#a5e8479d314bc290cf06312a7d932b4ca). However, it would be nice to provide some option on `CesiumRuntimeSettings` to allow a user to clear this cache, both in-Editor and at runtime (e.g., on application exit).

Related: https://github.com/CesiumGS/cesium-unreal/pull/1825

Start with CesiumRuntimeSettings and inspect how the cache is currently controlled through maxItems in the Editor and at runtime. Review the existing C++ cache-clearing approach and the related cesium-unreal pull request for expected behavior. Done means users can explicitly clear the cache in the Editor and at runtime, including on application exit.

Written by the indexing model from the issue text.