piv: permit use of shared smartcard connections

[ericchiang]

Updates #47

[DavidBuchanan314]

Is there anything in particular blocking this from getting merged? I think this does exactly what I need 🙏

Edit: I see your comment here about testing on newer yubikeys - I have a 5C that I can use for testing, if that's of any help #160 (comment)

[tianon]

I needed this, so I took the liberty of rebasing and updating it (which was a little non-trivial, especially in piv_test.go), and I think I've got the same spirit as all the changes here (many of which I had to reimplement green). 🙇

You can find my rebased version at https://github.com/tianon/piv-go/tree/shared (v2...tianon:piv-go:shared, or go-piv:ca0958e...tianon:aee16e4 for the permalink).

I've done some testing with it and it resolved a lot of issues I was having (I'm implementing something that can do ECDSA signatures via a YubiKey against this library, and before applying this, any concurrent attempts to sign would fail, and with this change I can happily make seemingly as many concurrent requests as I want and they just stack up, which is exactly how I want it 💖).

FWIW, I've been testing on a YubiKey 5C, although I will admit I have not run those updated tests (only verified that they compile correctly).

[tianon]

(Also, to be clear to Eric, if it would be helpful for me to submit that explicitly as a separate PR, I'm happy to, but I didn't want to trample you here unless you want that! ❤️)

Edit (2025-08-22): now opened at #182