Implement NFC

[msirringhaus]

Very heavily based on #105, with the following additions:

• Fleshed out the APDU-calls
• Added select_fido2() to discover both U2F and FIDO2-only devices
• Made NfcDevice derive Clone and other minor tweaks, so we can easily use it in credentialsd
• Added the build with both NFC-features to the github CI (including the installation of libnfc et al.). I kept this separate from the original, un-featured build, so we can be sure the former works even without libnfc installed.

Open questions:

• Biggest question to me is, how to handle the multiple backends and multiple devices. Since we can't run the "blink and wait for user presence"-scheme with NFC as we do with USB, I'm leaning towards simply always returning the very first found NFC-device instead of a list of found devices and use that. With this, we could get rid of the duplication-problem. I'm not sure many setups would include more than one NFC-reader (with different devices on there).
• The cancel-handle is currently unused, and I'm not sure if we can use it anywhere at all.
• No testing yet. Not sure how easy that would be to add.
• Given that credentialsd uses a poll-mechanism to find USB-devices (simply looping over list_devices()) as well, I did not investigate to get a blocking device discovery.
• For testing, I hacked my local credentialsd-repo by simply replacing hid with nfc in the USB-handler. Thus, I needed a dummy blink_and_wait_for_user_presence()-implementation. This should probably be removed, esp. if we go for the "return first device"-scheme mentioned above. But doing that, I was able to successfully use NFC-devices with Firefox.

[AlfioEmanueleFresta]

Thank you very much for picking this up @msirringhaus!

[AlfioEmanueleFresta]

Biggest question to me is, how to handle the multiple backends and multiple devices. Since we can't run the "blink and wait for user presence"-scheme with NFC as we do with USB, I'm leaning towards simply always returning the very first found NFC-device instead of a list of found devices and use that. With this, we could get rid of the duplication-problem. I'm not sure many setups would include more than one NFC-reader (with different devices on there).

This seems a reasonable approach.

If in the future multiple backends scenarios arise, contributions to make this configurable will be welcome!

[AlfioEmanueleFresta]

Thank you very much @msirringhaus, this looks great!