Support externally-controlled DIDs with verification method keys in Guardian

[anvabr]

Problem description

Current mode of Guardian operation assumes local storage of DIDs keys. There is much flexibility wrt what technology is used to store these keys (Hashicorp is often used in UAT/PROD, local DB usually used for local dev instances, etc) however in all cases the keys are controlled by the Guardian. In many use-cases DIDs for Standard Registry is a very important and a heavily guarded asset, in PROD such organisations would likely be reluctant to deploy the main keys into Guardian.

Requirements

Introduce a workflow into the Guardian where a DID Controller would introduce a dedicated verification method into the main DID for which the private key would be stored and managed by a Guardian instance. This way Guardian would only be able to control the specific verification method's key, but not the rest of the DID.

Definition of done

• Functionality implemented
• Documentation updated accordingly
• Demo videos etc updated accordingly

Acceptance criteria

It is possible to import an existing private key of the DID's verification method into Guardian such that it would then seamlessly work as usual for all the tasks as before.