Support data uri as resource content

I would like to add support for data uri as resource content. The idea is for the content object to be:
```json
{
"id": "{dataUri}"
}
```
Nothing is preventing this as the content is arbitrary json, however I'm wondering if this special case could have a specific resourceType defined, by which the content is expected to only contain a data uri. This could be valuable to secure inline images or other inline data.

My current idea is that a `resourceType` registry could be defined as an annex to the specification for protected resourceTypes, and have `DataURL` as one of the registered resourceType.

Start by reading the DID Attested Resources specification and its resource content model. Determine whether a protected resourceType registry and a DataURL entry are in scope; done would require a settled specification decision and clearly defined resulting changes.

Written by the indexing model from the issue text.