Simplify code for checking config availability and SRE deployment status

[craddm]

✅ Checklist

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🚦 Depends on

⤴️ Summary

Simplifies the code for checking which configs are available on the SHM and which SREs are deployed. No longer needs to load each stack, and thus is much faster.

🌂 Related issues

🔬 Tests

Tested locally

[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
config.py
Project Total

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

[JimMadge]

Very nice, good use of the Pulumi SDK to avoid reinventing the wheel.