fix(ssh): add support for interactive ssh keys when adding to SSH agent

[pascalbreuninger]

Separates between SSH keys that can be used as-in and the ones that
require additional input from the user like a passphrase.
In the latter case we now check if Stdin is interactive and prompt users
for input if it is.

solves #1546

[bkneis]

LGTM! Just one nit and a question on timeout length

[pascalbreuninger]

FYI: Waiting for the creator of ticket #1546 to try this PR before merging into main

[MartinLoeper]

Thanks so much for picking this up so quickly @pascalbreuninger.
I checked out your branch, built it and tested it. It just works! :)

There was another issue I had to fix though and I want to hear your opinion on it:
I had to run eval $(ssh-agent) before running devpod.
The reason being gpg-agent is apparently in read-only mode under certain circumstances.
This is an issue as devpod tries to add a bunch of ssh keys to it leading to errors like could not add identity: agent refused operation. Is there a way to make devpod users aware of it and that they should fix their ssh-agent configuration to make it writable?

[pascalbreuninger]

Closing this in favor of #1559 because I've accidentally pushed from my main branch and this is really annoying to develop against :D