Use unique keypair per credential

[emlun]

Companion PR of:

• Support unique, per-credential key pairs wallet-backend-server#66

This is "phase 1" of wwWallet/wallet-ecosystem#62.

This restructures the encrypted data contents from a flat set of attributes of a single key into a map of key identifiers to key attributes (key ID, DID, public key and wrapped private key), and removes the initial key generation on registration. Instead, createIdToken and generateOpenid4vciProof now generate a new key pair and add it to the encrypted data. Consequently, the encrypted data needs to be updated on the server after each such change. The stateless keystore module simply returns the updated encrypted data, and the stateful LocalStorageKeystore returns the updated encrypted data and a CommitCallback to invoke when the encrypted data has been successfully updated on the server. When called, the CommitCallback writes the new encrypted data to localStorage and the new encryption key to sessionStorage.

This depends on:

• Rename UserEntity.webauthnUserHandle to uuid and re-key local database from numeric ID #303
• Eliminate session key #304
• Extract function api.updatePrivateData and improve errors on X-Private-Data-ETag mismatch #305
• Replace function keystore.rotateMainKey with updatePrivateData #306