[refactor][kubectl-plugin] make --node-type an enum flag

[davidxia]

in the kubectl ray log command.

This is more complex but has several advantages.

• better help and error messages, e.g. shows the default "all" value in the
  help messages
• more type-safe
• can set the default value of "all" more easily
• supports shell completion

References

• https://stackoverflow.com/a/70541824/553994
• https://applejag.eu/blog/go-spf13-cobra-custom-flag-types/

Before

$ kubectl ray logs -h
...
      --node-type string               type of Ray node from which to download log, supports 'worker', 'head', or 'all'
...

$ kubectl ray logs dxia-test
No output directory specified, creating dir under current directory using resource name.
Command set to retrieve both head and worker node logs.
Downloading log for Ray Node dxia-test-default-group-worker-dgxsl
Downloading log for Ray Node dxia-test-head-xnh9r

$ kubectl ray logs dxia-test --node-type foo
No output directory specified, creating dir under current directory using resource name.
Error: unknown node type `foo`

$ kubectl ray logs dxia-test --node-type WORKER
No output directory specified, creating dir under current directory using resource name.
Command set to retrieve only worker node logs.
Downloading log for Ray Node dxia-test-default-group-worker-dgxsl

After

$ kubectl ray logs -h
...
      --node-type enum                 type of Ray node from which to download logs, supports: 'worker', 'head', or 'all' (default all)
...

$ kubectl ray logs dxia-test
No output directory specified, creating dir under current directory using resource name.
Command set to retrieve both head and worker node logs.
Downloading log for Ray Node dxia-test-default-group-worker-dgxsl
Downloading log for Ray Node dxia-test-head-xnh9r

$ kubectl ray logs dxia-test --node-type foo
Error: invalid argument "foo" for "--node-type" flag: must be one of "all", "head", or "worker"

$ kubectl ray logs dxia-test --node-type WORKER
No output directory specified, creating dir under current directory using resource name.
Command set to retrieve only worker node logs.
Downloading log for Ray Node dxia-test-default-group-worker-dgxsl

Checks

• I've made sure the tests are passing.
• Testing Strategy
  • Unit tests
  • Manual tests
  • This PR is not tested :(

[davidxia]

What my fish shell looks like when I press after the end of --node-type after enabling shell completion. Tabbing scrolls through the options and populates my command. Very nice!