tpm2key.asn1 parent identification

Hi,

In practice, while implementing tpm2sh and its self-contained TPM emulator
called "MockTPM", I've noticed that 'tpm2key.asn1.' has a major bottleneck,
but luckily it is easy to squash.

Parent handle should never be persisted, as it defies the existential
reason of having a file format in the first place.

To address this issue I just added couple of optional fields to TPMKey:

  parentName   [6] EXPLICIT OCTET STRING OPTIONAL,
  parentPubkey [7] EXPLICIT OCTET STRING OPTIONAL

By persisting this information TPM2_GetCapability + TPM2_ReadPublic can
be used to acquire an appropriate handle.

I'd highly recommend to add this quirk to anything that processes this
ASN.1 format.

BR, Jarkko