feat: Use multiple yubikeys at same time

[ronantakizawa]

Description (e.g. "Related to ...", etc.)

Use multiple yubikeys at same time (Key generation, repo generation, target signing). Resolves issue #244

Code review checklist (for code reviewer to complete)

• Pull request represents a single change (i.e. not fixing disparate/unrelated things in a single PR)
• Title summarizes what is changing
• Commit messages are meaningful (see this for details)
• Tests have been included and/or updated, as appropriate
• Docstrings have been included and/or updated, as appropriate
• Changelog has been updated, as needed (see CHANGELOG.md)

[renatav]

I remember having to build pyscard from source a while back, but at some point, it was no longer necessary. If this is a temporary workaround, please create an issue and provide some details, so that we don't forget about it.

[renatav]

I am currently unable to test this since I am not able to insert two yubikeys at the same time, but based on the code, it looks like you have to manually enter each of the yubikey's serial number. This is probably less user-friendly compared to having to remove and insert the devices. A bigger issue, however, is that we want to be able to sign metadata using YubiKeys plugged into our server. So, this should iterate over all inserted YubiKeys and simply try to sign using all of them, instead of asking for any input. So, let's say that you have timestamp and snapshot keys inserted and you need to sign timestamp metadata. We need to automatically detect the key that can sign the timestamp metadata. At the moment, the user will have to enter pins too, but we don't need to address that in this PR.

[renatav]

I'll merge this into a feature branch, to move this code off of the fork. Makes sense at first glance, but I am not able to test this since I am not able to plug in two YubiKeys at the same time.