Have kubelogin update itself

[ghost]

New versions of kubelogin are required to work with newer infrastructure (new configs, new systems, etc). Investigate options to have kubelogin check whether it is up-to-date and download the newest version if available.

Quick google results include:
https://github.com/inconshreveable/go-update
https://github.com/rcrowley/goagain

Obligatory StackOverflow: https://stackoverflow.com/questions/27823042/is-it-possible-for-a-go-binary-to-reload-itself

[ghost]

Brief test examples show that Windows locks binaries while executing, meaning this needs a secondary program to update kubelogin.exe

We could include a subpackage called updater and have the resulting updater check its own version, then download a newer copy to replace the existing kubelogin binary.

1. Run kubelogin
2. Kubelogin detects it is out of date
3. Kubelogin spawns updater (optional: passes all args to updater)
4. Kubelogin exits, reporting that it is updating itself
5. Updater downloads new kubelogin, replaces existing kubelogin executable
6. (Optional: relaunch kubelogin with args)

I'm not sure if we want to "error out" with a message asking someone to run kubelogin again, but that could be a good first step.