Read crates whose metadata contains JSON-LD value objects

[simleo]

Adds minimal support for JSON-LD value objects: now an entry like:

"description": {
    "@value": "A collection of my pictures",
    "@language": "en"
}

is treated like:

"description": "A collection of my pictures"

whereas previously we crashed with:

ValueError: no @id in {'@value': 'A collection of my pictures', '@language': 'en'}

Note that the validator has recently added support for value objects in crs4/rocrate-validator#116.