Unify launch methods

**Problem**

PySTK is able to launch:

- STKDesktop
- STKEngine
- STKRuntime

Right now, each one requires its own import and options. This differs from other libraries, which usually provide the following user experience:

```python
from ansys.product import launch_product

product = launch_product(...)
...
product.close()
```

**Solution**

The ecosystem is adopting [ansys-common-tools] as the way to unify the user experience. This library provides utilities to ease the implementation of product launchers.

We could implement this as an extension. This way, the `ansys.stk.core` can remain dependency free and users can still use the current `STK{Desktop,Engine,Runtime}` objects.

[ansys-common-tools]: https://github.com/ansys/ansys-tools-common