[waltid-cli] When a DID is created also the automatically generated Private Key should be stored in the file-system

[philpotisk]

Is your feature request related to a problem? Please describe. when calling waltid did create without specifying a key, a key is automatically generated and embedded in the DID, but the private key is currently not stored, which makes the DID more or less useless.

Describe the solution you'd like The Private Key should also be stored in the file system (same way as: waltid key generate does it).

Describe alternatives you've considered It is not a big issue, since the key can be anyway generated beforehand and used in the did create command.