* #9 Implement routes to list, get and delete execution environments.
A refactoring was required to introduce the ExecutionEnvironment interface.
* Fix MR comments, linting issues and bug that lead to e2e test failure
* Add e2e tests
* Add unit tests
The TestCreateOrUpdateEnvironment function would previously use
the python:latest Docker image in its execution environment request.
However, this lead to pull rate limiting by Docker Hub in our CI.
We previously didn't really had any structure in our project apart
from creating a new folder for each package in our project root.
Now that we have accumulated some packages, we use the well-known
Golang project layout in order to clearly communicate our intent
with packages. See https://github.com/golang-standards/project-layout
