feat: replace @type with type and roundTrip testing

[wolf4ood]

WHAT

This PR replaces the reference to @type with type in the example and the required field section.

Additionally added the round trip testing of the examples JSON-LD processing + schema validation

Closes #88

How was the issue fixed?

Briefly state why the change was necessary.

More context

List other areas that have changed but are not necessarily linked to the main feature. This could be naming changes,
bugs that were encountered and were fixed inline, etc.

[arnoweiss]

This is great. One question: Are we actively deciding to keep the option between @type and type in the schemas? I'd feel more comfortable if we add "type": "@type" to the dcp context and remove @type from the schemas. That may however break non-json-ld-aware applications.

[wolf4ood]

We already to the term redefinition for type to @type

https://github.com/eclipse-dataspace-dcp/decentralized-claims-protocol/blob/main/artifacts/src/main/resources/context/dcp.jsonld#L7-L9

It's fine for me to remove the @type from the JSON schema as long we clearly state that we do not support it.

Which app would break with type?

[jimmarino]

@wolf4ood is there anything additional you need to do before this is merged?

[arnoweiss]

I'll add another issue for removing the @type option from the schemas.