Pnut crosspost detail with source

crosspost-with-source.md

I believe all but canonical_url would be optional. But if including source, both url and name would be required. If including user, id would be required.

{
    "type": "io.pnut.core.crosspost",
    "value": {
        "canonical_url": "https://some-matrix-service.org/howeverthisgoes/123",
        "user": {
            "id": "@alice:example.com",
            "username": "alice",
            "name": "Alice Margatroid (Optional)",
            "avatar_image": "https://matrix.org/SDGdghriugerRg"
        },
        "source": {
            "url": "https://matrix.org",
            "name": "Matrix"
        }
    }
}