Support compressed format for EC2SigningKey

[vikram-gaur]

We should support compressed format. Seems like we only support full coordinates. From the RFC:

   +----------+------+-------+-------------+-----------------------------------+
   | Key Type | Name | Label | CBOR Type   | Description                       |
   +----------+------+-------+-------------+-----------------------------------+
   | 2        | crv  | -1    | int / tstr  | EC identifier - Taken from the    |
   |          |      |       |             | "COSE Elliptic Curves" registry   |
   | 2        | x    | -2    | bstr        | x-coordinate                      |
   | 2        | y    | -3    | bstr / bool | y-coordinate                      |
   | 2        | d    | -4    | bstr        | Private key                       |
   +----------+------+-------+-------------+-----------------------------------+