First implementation of PIN handling

[sosthene-nitrokey]

The first implementation would probably not need to be complete can only have the PIN creation and checking in place.

API

• Pins would be represented by a pub struct PinId(pub u8) that is set by the client (unlike the KeyIds which are random.
• Pin creation would allow setting the maximum number of retries.
• Pins can be deleted.
• Pins can be changed with set pin (initially this doesn't require authenticating it before)

Implementation

• Use PIN Ids in a /client_namespace/pins/<pin_id> file. No file = PIN does not exist
• Pin stored with SHA-256 salted
• 1 Salt per Pin