Simplify GuacamoleUsers lookup

[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

Currently GuacamoleUsers relies on making a PostgreSQL query from an admin machine to the locked-down PostgreSQL user database. By exposing a less-sensitive read-only endpoint, we could remove the need to access the database directly.

🚂 How could this be done?

Possibly a function app or a lightweight API endpoint?