Create Guacamole database with Pulumi

[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

Create the initial Guacamole database with Pulumi instead of SREProvisioningManager.

🚂 How could this be done?

Use the dbforpostgresql Provider.