MVP of DID integration + Key Management API

[dmitrizagidulin]

1. On account creation, also create a Decentralized Identifier (DID) and register it with an identity-focused blockchain (such as Veres One)
2. In order for that to be useful, the private keys to that DID also need to be stored somewhere. So, either:
   a. Generate and store corresponding private keys in a keys/ container in the user's data space, under access control. Or
   b. Store the keys in a Credentials Handler API powered wallet. Or both.
3. (Optional) Consider adding a basic /sign API endpoint. So that a user/app developer can send an arbitrary JSON document to that endpoint to be signed by the current logged in user.