[k8s] Support Exec-based Auth

[kyuds]

Fixes #4379

Need more testing and a lot of feedback please. The main hurdle with supporting exec-based auth is that when using exec-based auth we cannot use incluster configurations at all (anything that uses incluster auth without a proper RBAC role will result in a lot of logs saying that localhost:8080 is inaccessible).

Currently tested on:

1. Start jobs controller on GKE and submit job to the same cluster (this is where the incluster issues come into play)
2. Start jobs controller on AWS EC2 and submit job to GKE cluster

Need to test:

1. Whether this works on EKS (main problem will be from having the correct PATH env var set. The gke auth plugin had a similar issue and as a temporary workaround used a sudo symlink, but this could be better. Being able to set a proper PATH env var will be a more general and workable solution)

Tested (run the relevant ones):

• Code formatting: install pre-commit (auto-check on commit) or bash format.sh
• Any manual or new tests for this PR (please specify below)
• All smoke tests: /smoke-test (CI) or pytest tests/test_smoke.py (local)
• Relevant individual tests: /smoke-test -k test_name (CI) or pytest tests/test_smoke.py::test_name (local)
• Backward compatibility: /quicktest-core (CI) or pytest tests/smoke_tests/test_backward_compat.py (local)

[cg505]

We may need some hard-coded whitelist of supported exec providers - some kubernetes users may be rolling their own exec-based auth that won't work from the remote VM. Though maybe it's fine for it to just fail in that case.

[kyuds]

We may need some hard-coded whitelist of supported exec providers - some kubernetes users may be rolling their own exec-based auth that won't work from the remote VM. Though maybe it's fine for it to just fail in that case.

I think we should fail, but alert users of supported exec providers through sky check. For exec-based auth to work, say for GKE, users need to enable both GCP AND K8S. This requirement is already logged on sky check, so we could also provide a list of available exec methods there.