Add Public Credential support

[JelleMillenaar]

Add support and standardize a service endpoint that links to a publicly available Verifiable Credential (VC), published to an IOTA address. This address can be resolved to retrieve a VC and can be verified. The public VC should also have its own type made available within the DID Document as filtering and searching for public credentials within a DID Document should be done with as few queries as possible.

This feature is targeted at Identity for Organization, Things, and Objects. It should not be utilized with any data that can be considered Personal Identifiable according to the General Data Protection Regulation (GDPR) and other privacy laws worldwide. As such, this feature should not be used with DID Documents representing people. It is ideally used for Organizational credentials such as certifications or used for Identity of Things for similar certifications, specifications, or proof of origin.

• Determine specs of the feature
• Implement Service Endpoint
• Standardize Service Endpoint
• Document very explicity the dangers of using this with Identity for People (GDPR compliance)
• Add WASM bindings
• Add examples

Internal design document: https://docs.google.com/document/d/1VE0jtPsd_9Ewtfj53pdHPJXzwS3lmR0yd4Uk0dpNjzM/edit

[eike-hass]

Dependent on #1283