Add Json-Ld context, cleanup message types, add proper context URLs

[jimmarino]

This PR adds the IATP Json-Ld context, cleans up missing message type information, and fixes incorrect context URLs. Specifically:

• Introduces a Json-Ld context that can be (optionally) used for runtimes to perform Json-Ld expansion and basic validation
• Renames the types attribute to type on CredentialRequestMessage so it is consistent throughout all documents and with W3C standards
• Replaces inconsistent property naming where _ was used instead of Camel Case (e.g. presentationDefinition instead of presentation_definition
• Renames types to credentialTypes on IssuerMetadata since renaming it totype would clash with the ODRL definition of type as @type, which would cause Json-Ld expansion to add the value to the IssuerMetadata @type property.
• Uses iatp as a prefix for consistency
• Uses the URL https://w3id.org/tractusx-trust/v0.8 for consistency

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Linked Issues

Closes #31