crypto: Native SPKAC support

Implements new class 'Certificate' within crypto object
for working with SPKAC's (signed public key & challenge)
natively.

bool Certificate::verifySpkac(buffer)
buffer Certificate::exportPublicKey(buffer)
buffer Certificate::exportChallenge(buffer)
crypto: Native SPKAC support

Implement new class 'Certificate' within crypto object for
working with SPKAC's (signed public key & challenge)
natively.

bool Certificate::verifySpkac(buffer)
buffer Certificate::exportPublicKey(buffer)
buffer Certificate::exportChallenge(buffer)