Specify format for HDK metadata records associated with attestations

[sander]

With each attestation, the Wallet Instance typically needs to be able to obtain:

• the device public key
• the associated seed
• the path to the HDK, where each edge can be represented by either:
  • an index, or
  • an ARKG key handle

We may keep some of these data implicit, relying on the ARKG key handle's MAC.

Like in BIP32, we may choose to standardise the representation of the metadata to associate. Note that this cannot completely be included in the attestation's issuer-sealed data, since that would introduce new correlation handles.

[sander]

Relates to #86