ruta-core is unable to access extension from ruta-core-ext in OSGI context

**Describe the bug**
ruta-core-ext contains some extension functions that need to be enabled e.g. as

```
createEngineDescription(
RutaEngine.class,
RutaEngine.PARAM_ADDITIONAL_EXTENSIONS, StringOperationsExtension.class);
```

However, the ruta-core bundle has no optional import of the required packages from ruta-core-ext and there is also no alternative such as an SPI which would allow discovery.

**To Reproduce**
Steps to reproduce the behavior:
1. Try running a Ruta engine which uses an extension in an OSGI context

**Expected behavior**
The simplest solution would probably be adding an optional import for the packages of `ruta-core-ext` to the `ruta-core` bundle.

**Please complete the following information:**
- Version: 3.3.0