Allow missing algorithms

[robin-nitrokey]

The algorithm field is optional, see RFC 8152 § 7:

   COSE_Key = {
       1 => tstr / int,          ; kty
       ? 2 => bstr,              ; kid
       ? 3 => tstr / int,        ; alg
       ? 4 => [+ (tstr / int) ], ; key_ops
       ? 5 => bstr,              ; Base IV
       * label => values
   }

   alg:  This parameter is used to restrict the algorithm that is used
      with the key.  If this parameter is present in the key structure,
      the application MUST verify that this algorithm matches the
      algorithm for which the key is being used.

Fixes: #3

Depends on:

• Refactor COSE serialization and deserialization #4

Ported from:

• Make COSE deserialization more robust ctap-types#8