Feature/add choice parameter and raise an exception if fetched value is not within

[Swannbm]

This PR addresses issue #193 (specifying allowed options)

Use case 1:

We don't want to hide warnings because a log-reading probe catches them and triggers an alert.

env = Env(LOGGING_LEVEL="EXCEPTION")
LOGGING_LEVEL = env.str("LOGGING_LEVEL")  # Ok
LOGGING_LEVEL = env.str("LOGGING_LEVEL", choices=["DEBUG", "INFO", "WARNING"])  # raise ImproperlyConfigured

Use case 2:

Define a set of execution environments with dedicated settings, and ensure the app is running in one of them.

env = Env(ENVIRONMENT="prod")
ENVIRONMENT = env.str("ENVIRONMENT")  # Ok
ENVIRONMENT = env.str("ENVIRONMENT", choices=["local", "qa", "staging", "production"])  # raise ImproperlyConfigured

Solution proposed:

• The use cases considered only environment variables of type str.
• Add a choices parameter to Env.str() to allow the user to define allowed options.
• Ensure that the fetched value is included in the provided choices list when the choices parameter is set.

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I have limited the scope of work, but I can extend the mechanism to some other fetching methods if you see use cases...

Let me know if some corrections are needed.