definitions for private information

[OR13]

https://github.com/w3c-ccg/universal-wallet-interop-spec/pull/25/files#diff-9c6d9b71546e84d84bee622bc124592cR43

@msporny if these terms are not going to be defined in this vocabulary, then we (universal wallet work item authors) will need to defined them in another vocabulary.

I'd prefer we use security vocab html to define them, but I would live with them not being in the same context....

For example:

https://w3id.org/security/v3 -> https://w3id.org/security#publicKeyJwk https://w3id.org/security/v3-private -> https://w3id.org/security#privateKeyJwk or https://w3id.org/wallet/v1-> https://w3id.org/wallet#privateKeyJwk