Add example for using the exec endpoint with stdin

[philipsd6]

As noted in #174, the synchronous version of this package has the pod_exec.py example, but only the non-interactive part of that is re-implemented in examples/example3.py.

Is it possible to implement the interactive version with this library as well? My use case is to start psql in the container and execute various ddl statements from an operator (based on kopf) and emit the responses.

[tomplus]

Sure, it's possible but it has to be implemented first #12

[herman-ai]

@tomplus is #12 being actively worked on? I would also like this feature for my project.

[tomplus]

@herman-ai I'm not working on it right now, feel free to contribute if you want.