Switch to new control-plane API

[thomasht86]

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Took the opportunity to clean up a bit for a more consolidatet http-client usage.
Think this will also remove instability in integration tests (where http.connection was used previously)

• Use new control plane-API
• Hardcode dev region (similar to vespacli)
• Switch to httpx-client for more unified usage
• Add common user-agent header for all clients with version
• Add integration tests for user-agent header

[thomasht86]

This should be good now.
Spent some time making the tests pass, and think I was able to fix what caused sporadic integration test failures.
Now, all the http-connections used everywhere are subjected to the retry-policies, and use either requests or httpx