Add git-sourced Smart Tests as examples + update hotrod images to point to latest

[foxish]

1. This PR also makes hotrod-istio the default namespace where the sandboxes are created and requires setting hotrod-devmesh if that's where you want to create the sandbox (inverse of current behavior)
2. This also removes the legacy signadot-proxy based integration tests from all but one location (route svc) since it's just serving as an example now.
3. We don't yet have label filtering during test execution, so, all sandboxes run all the smart tests.
4. Remove the quickstart tags and point the images to latest (this change can only be merged with the new quickstart)

TODOs:

• update quickstart
• update playground cluster to reference hotrod:latest

[daniel-de-vera]

@foxish, the changes LGTM, but:

• Can we introduce a dummy change somewhere in the code (in location or frontend) to trigger a smart test run? I would like to see it in action 😊 (and confirm it works as expected)
• We can't merge this PR until we complete the TODOs, otherwise we would be breaking the playground clusters:

https://github.com/signadot/signadot/blob/5de3cbf78b287ecf5f1c28b1481ccb51b068e59f/playground/deploy/base/vcluster-bootstrap/baseline/kustomization.yaml#L3-L4

Maybe an alternative is not removing the quickstart overlay here?

[signadot]

Sandbox pr-274-frontend created

Routing Key: 4pcr50bfuyv4q
Last Updated: 2025-04-09 23:19 UTC

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[foxish]

The action passes @daniel-de-vera - this is now ready for another look. I canceled the executions because of headroom issues.

[daniel-de-vera]

LGTM