Add support for multiple certificate sets.

[tuxmaster5000]

Until now, only one set ( 2 keys encryption/sing) can be stored on the device.
But for example using it for Thunderbird with multiple accounts it will not be enough.
I think the minimum capacity must be 4 - 6 sets (8 - 12 key pairs) for an real works usage.

[daringer]

Yes, generally we are planning to have multiple identities available for the OpenPGPCard functionality. Although I cannot give an ETA for that feature currently.

[szszszsz]

Connected: Nitrokey/opcard-rs#51