Refactor Azure authentication classes

[jemrobinson]

✅ Checklist

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🚦 Depends on

n/a

⤴️ Summary

• Reduce Azure-connected classes to two: AzureSdk and GraphApi
• Use our own DeferredCredential class which can cache tokens to reduce external API calls
  • use of cached tokens avoids the "stalling bug" which was otherwise triggered by making an AzureCLI call during the finally clause of a try statement
• AzureSdkCredential provides authentication for AzureSdk using the Azure CLI
• GraphApiCredential provides authentication for GraphApi using a browser device-code
• Fixes a bug in http_get where we were ignoring additional pages for paged responses.
• N.B. this removes the confirm dialogue which was applied for AzureCLI logins but not for Browser logins. If we want this, I can easily add it to both.

🌂 Related issues

Closes #2001
Closes #1980

🔬 Tests

Stale

[JimMadge]

Still a few gaps in the coverage in credentials.py, graph_api.py and azure_sdk.py.

Things like getting credentials from files and exception handling.

[jemrobinson]

@JimMadge : I think this now tests all the functions under external/api. Are you happy to merge with these changes?

[JimMadge]

Just one comment, could leave for another PR if we open an issue.