Improve kubectl plugin

[alexkursell]

Based on feedback in #3831, I've made the following additions to the plugin

• Add --service and --deployment flags where appropriate for situations in which the ingress-nginx service or deployment names are not the defaults.
• Add logs command to display the logs of an ingress-nginx controller
• Add exec command to execute a command inside the controller
• Add ssh command to get a shell inside the controller.
• Add the ENDPOINTS column to ingresses to show the number of endpoints for the backend service (or N/A if it's an ExternalName). This count currently ignores some of the subtleties of endpoints where not every endpoint in a service is actually serving the particular port the ingress is using, but it's good enough for a rough count.

To get this merged, I feel like there are a few things left to consider:

• Properly implementing exec and ssh (and logs too, with all of the flags it should support) might require duplicating uncomfortably large parts of kubectl functionality. It might be better to have those commands "fall-through" and just call kubectl directly behind the scenes with the correct pod name. I don't think this plugin will ever be used in a context where kubectl doesn't exist, but this warrants some discussion. edit exec, ssh, and logs have been replaced with kubectl fall-through
• Add all of the supported kubectl logs flags to logs
• Refactoring. main.go in particular is getting uncomfortably large and should probably be broken up into multiple files

Some things that should probably get done before the 0.24.0 release, but maybe not necessarily in this PR:

• Adding e2e (and possibly unit) tests
• Writing documentation

[aledbf]

Properly implementing exec and ssh (and logs too, with all of the flags it should support) might require duplicating uncomfortably large parts of kubectl functionality.

Yes, please use kubectl and remove the client-go code. This not only removes code from this repo but is consistent with the kubectl binary the user is already using.

[alexkursell]

@aledbf, @ElvinEfendi I think this PR is about ready feature-wise. Let me know if you have any concerns or any more features you want me to add.

[aledbf]

/approve

[ElvinEfendi]

/lgtm

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