Add entrypoints for pipx

[pitrou]

Is your feature request related to a problem? Please describe.

Since storage-testbench is a Python project providing a single executable command, it would be nice if it could be managed using pipx. It would make it easier to run without having to know which Python was used to install it.

Describe the solution you'd like

The pipx docs describe how to add entry points to setup.py in order to support running with pipx:
https://pypa.github.io/pipx/how-pipx-works/#developing-for-pipx

Describe alternatives you've considered

What we're doing right now, for Apache Arrow, is install the testbench in a Python environment that's also used for unrelated things. It creates coupling between different pieces of our CI, and also requires passing/finding the proper Python executable even from non-Python tests, such as in the following C++ code:
https://github.com/apache/arrow/blob/532ab464db39da0e6b03e4b046355e7bf499730a/cpp/src/arrow/filesystem/gcsfs_test.cc#L92-L133

[pitrou]

cc @raulcd @coryan