[k8s] Remove SSH jump pod for port-forward mode

[romilbhardwaj]

Closes #3566. SSH jump pod is not required when using port-forward mode. This PR directly kubectl port-forwards to the relevant pod.

Reduces SSH time by ~25% (1s) on a GKE cluster.

Also lays the groundwork for easy switching between kubecontexts/kubeconfigs while retaining SSH functionality (requested by user).

TODOs -

• Need to figure out multi-node support with this, since each worker will now require a unique SSH ProxyCommand. Current SkyPilot plumbing assumes a common ProxyCommand across all workers.

Benchmarks

======= This branch - After removing SSH Jump pod =======

1: ssh test ls
            Mean        Std.Dev.    Min         Median      Max
real        3.014       0.277       2.525       3.125       3.309
user        0.020       0.002       0.016       0.020       0.022
sys         0.008       0.001       0.005       0.008       0.009

multitime -n 5 sky launch -y --cpus 1 --cloud kubernetes task4.yaml
1: sky launch -y --cpus 1 --cloud kubernetes task4.yaml
            Mean        Std.Dev.    Min         Median      Max
real        60.359      2.530       57.249      61.003      63.215
user        4.299       0.492       3.875       3.942       5.095
sys         3.770       0.325       3.492       3.563       4.360

======= Master - with SSH Jump pod =======

===> multitime results
1: ssh test ls
            Mean        Std.Dev.    Min         Median      Max
real        3.960       0.386       3.310       4.099       4.454
user        0.019       0.002       0.017       0.018       0.022
sys         0.007       0.000       0.006       0.007       0.008

multitime -n 5 sky launch -y --cpus 1 --cloud kubernetes task4.yaml
1: sky launch -y --cpus 1 --cloud kubernetes task4.yaml
            Mean        Std.Dev.    Min         Median      Max
real        63.814      2.132       61.270      63.385      66.609
user        3.791       0.155       3.535       3.854       3.976
sys         3.590       0.062       3.509       3.573       3.670

Tested (run the relevant ones):

• Code formatting: bash format.sh
• Smoke tests

[romilbhardwaj]

Closing in favor of #3657.