Implement struct for retrieving DID document

[miguelnietoa]

ℹ️ This issue is part of Epic #81

Objective

Implement a structure within the DIDContract that facilitates the retrieval of public DID documents. This structure should enable the display of relevant DID document information to the contract owner. Additionally, it should provide capability invocations that allow authorized parties to read specific credentials within the system.

Checklist

• Display of relevant DID document information to the contract owner
• Provide capability invocations that allow authorized parties to read specific credentials within the system