workflows: add multicluster CI 3.0 workflow

[nbusseneau]

This was ported and adapted from cilium-cli as part the CI 3.0 initiative.

[nbusseneau]

Test run for this workflow here: https://github.com/cilium/cilium/actions/runs/752276326

[nbusseneau]

Alright so we have pod-to-pod and pod-to-service failing:

❌ 2/7 tests failed (0 warnings)

Failed tests: ❌ pod-to-pod: cilium-test/client-6c46bff775-d5s7k -> cilium-test/echo-other-node-86774f89b9-dsvds, ❌ pod-to-service: cilium-test/client-6c46bff775-d5s7k -> echo-other-node:8080 (ClusterIP), ✅ pod-to-service: cilium-test/client-6c46bff775-d5s7k -> echo-same-node:8080 (ClusterIP), ✅ pod-to-world: cilium-test/client-6c46bff775-d5s7k -> https://google.com, ✅ pod-to-host: cilium-test/client-6c46bff775-d5s7k -> 10.168.0.30, ✅ pod-to-host: cilium-test/client-6c46bff775-d5s7k -> 10.168.0.33, ✅ pod-to-pod: cilium-test/client-6c46bff775-d5s7k -> cilium-test/echo-same-node-f789dd8f7-4mqml

However this is the same as in cilium-cli repo: https://github.com/cilium/cilium-cli/actions/runs/631421886
This has been broken since 2~3 months 😅

[nebril]

Left one non-blocking comment.

[nbusseneau]

No idea why but this run was successful: https://github.com/cilium/cilium/actions/runs/756782113
Is it a flake? Will re-run to see. EDIT: still working 🎉 https://github.com/cilium/cilium/actions/runs/756897245
Only difference I think had a potential impact was switching to COS_CONTAINERD image type.

[nbusseneau]

Marking as ready to merge as Maciej actually already validated on behalf of ci-structure, he just did it before I switched the PR out of draft and ci-structure was added.