Support for EC keys, fix Attributes processing

[ya-mouse]

This change adds proper handling for EC keys (addressing #303) as well as proper RSA parameters calculation.

NOTE: For instance, openssl expects that EC params (curve OID) and points are in specific uncompressed and padded format. In Go encoding EC_POINTS be like:

case C.CKA_EC_POINT:
    encodedPoint, err := encodeECPoint(key.Curve, key.X, key.Y)
    ...
    setValue(attr, unsafe.Pointer(&encodedPoint[0]), C.CK_ULONG(len(encodedPoint)))

func encodeECPoint(curve elliptic.Curve, x, y *big.Int) ([]byte, error) {
        fieldSize := (curve.Params().BitSize + 7) / 8
        xPadded := make([]byte, fieldSize)
        yPadded := make([]byte, fieldSize)
        copy(xPadded[fieldSize-len(x.Bytes()):], x.Bytes())
        copy(yPadded[fieldSize-len(y.Bytes()):], y.Bytes())

        ecPoint := append([]byte{0x04}, append(xPadded, yPadded...)...)

        return asn1.Marshal(asn1.RawValue{
                Tag:   asn1.TagOctetString,
                Bytes: ecPoint,
        })
}

I faced this issue when implementing fake custom backend using regular PEM/DER files or passing signature produced by AWS KMS.
Perhaps, as later improvement we can try to instantiate specific signature from DER bytes and then repack params into expected format. Now this is offloaded to the backend.

The changes were checked with my custom backend using RSA2048 and ECC NIST-P256 keys via OpenSSL.

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[ya-mouse]

we should skip unknown attributes otherwise a tool that handles pkcs11 calls silently tries to interpret random values from the template: pkcs11-tool and p11-kit produces weird values for attributes when inspecting with pkcs11-spy.

[brandonweeks]

Can you please split the bug fix for attribute parsing into a separate PR?

[ya-mouse]

#389
#390

[ya-mouse]

was small to fit ECC P-256 digest value

[ya-mouse]

Added some missed attributes with default values that pkcs11-tool/p11-kit tools complains about.

[brandonweeks]

Thank you for the PR! Just to set expectations, I probably won't have an opportunity to review and test until the new year.