Validate SHM/SRE for user commands

[JimMadge]

✅ Checklist

• You have given your pull request a meaningful title (e.g. Enable foobar integration rather than 515 foobar).
• You are targeting the appropriate branch. If you're not certain which one this is, it should be develop.
• Your branch is up-to-date with the target branch (it probably was when you started, but it may have changed since then).

🚦 Depends on

⤴️ Summary

Add checks that Pulumi configuration for SHM/SREs exists.
This is a proxy for ensuring the infrastructure exists.
It should be more reliable that just checking if the names are used in the DSH configuration, and much lighter weight than checking for particular pieces of infrastructure (either using Azure API or looking through the Pulumi stack).

🌂 Related issues

Closes #1879

🔬 Tests

[github-actions]

Coverage report

Click to see where and how coverage changed

File	Statements	Missing	Coverage	Coverage (new stmts)	Lines missing
data_safe_haven/commands
users.py					124, 142, 226, 244
Project Total

This report was generated by python-coverage-comment-action

[craddm]

LGTM