CodeInterpreter should not require a region argument

**Is your feature request related to a problem? Please describe.**
To create a `CodeInterpreter`, a `region` string argument must be passed. I use boto3 elsewhere in my application, and rely on its default region resolution mechanics. Now I need to explicit configure a region, which has to come top-down from my application.

**Describe the solution you'd like**
I should be able to create a `CodeInterpreter` without passing a region. It should default to using boto3's resolution for the region. If no region is configured, boto3 will raise an exception when performing an API operation.

**Describe alternatives you've considered**
I can manually create a session and get the region from it, then pass that same session to `CodeInterpreter`. This isn't quite right though, since `region_name` may be `None`. I could check if it's None and raise an exception myself, but having to do all this is awkward. Though I can also see the argument for "explicit is better than implicit."

```py
session = boto3.Session()
code_interpreter = CodeInterpreter(session.region_name, session)
```

Beginne beim CodeInterpreter-Konstruktor und untersuche, wie seine region und boto3-Session verwendet werden. Überprüfe den im Issue beschriebenen default-region-Pfad und bestätige anschließend, dass das Erstellen von CodeInterpreter ohne region funktioniert und dass boto3 einen Fehler wegen einer fehlenden region weiterhin nur dann meldet, wenn eine API-Operation sie erfordert.

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