Limit scope to wallet key management

[sander]

With the refactoring as of e97d160, the scope of HDK could be reduced to specifying functionality for key management within a single wallet solution. The use cases could be specified from a holder perspective only. This simplifies the specification and clarifies the primary purpose.

Potentially blocking issues:

• [x] Specify how the holder shares the ARKG public key with the issuer.
• [x] Specify how the issuer shares the ARKG key handle with the holder.
• [x] Specify how a single parent key proof of possession is enough for batch derivation and issuance.

[sander]

After further discussion, focus on HDK more like BIP32, presenting two methods for efficient privacy-preserving EUDI wallet key management:

• HDK method to efficiently derive keys purely in the wallet, with clear separation between a seed (for one WTE), and plaintext paths
• Blinding method using a single WSCD private key

Example application: single-use credentials with ARKG.

[sander]

Done.