[FEATURE] Import authorized_keys public SSH Keys from GitHub username

[nelsonjchen]

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The device I'm flashing on may not have a copy of all my public keys. Or it might have just one of my public keys and I have other workstations. But I do know my GitHub username and that does have all or most keys from all or most of my workstations...

https://github.com/nelsonjchen.keys

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Be able to enter in a GitHub username and import all public SSH keys from GitHub as an option for pre-filling SSH keys when flashing.

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Everything else is work. I'm lazy. In all seriousness, maybe ssh-copy-id, but that's not available on all workstations and certainly doesn't solve the whole "stuff all my workstation keys from GitHub" into the flash issue. I guess curl https://github.com/nelsonjchen.keys >> ~/.ssh/authorized_keys might work too, but that'll have to be done interactively for all.

The other wild idea is SSH CA. That's a wholly different issue 😄 and not one I've seen examples like mentioned in Additional Context. And I don't think it's or would be very popular.

Additional context
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I've seen this in Ubuntu Server's installer and comma.ai openpilot's SSH setup UI. In both of those cases, "Import from GitHub" was extremely convenient. I suspect there could be other examples out there as well.

[lurch]

Duplicate of #374 ?

[nelsonjchen]

Wow! I was searching issues quite furiously and never would have guessed it was already at languishing PR stage.

[tdewey-rpi]

Tagging this for 2.0. This is a pretty powerful feature, and it will require some additional UI space to look natural inside Raspberry Pi Imager.