Export categories from functions module

[jemrobinson]

✅ Checklist

• I have searched open and closed issues for duplicates.
• This is a request for a new feature in the Data Safe Haven or an upgrade to an existing feature.
• The feature is still missing in the latest version.
• I have read through the documentation.
• This isn't an open-ended question (open a discussion if it is).

🍓 Suggested change

From here:

Wondering if it would make sense to make functions export categories like strings.

from data_safe_haven.functions import strings

strings.b64encode(...)

A bit like with the validators package.

🚂 How could this be done?

Update the code structure and test.