Separate subject validation from JSON-LD schema validation

[tdiesler]

Currently, validation assumes that credentials/presentations have to be in JSON-LD format

[tdiesler]

PR: #36

[tdiesler]

Rebased PR: #43

[tdiesler]

Could you please have a look at this PR? It is currently blocking nessus-didcomm integration with Apache Camel, which cannot have transitive dependencies on SNAPSHOTS

Please also consider publishing a tag to maven central. Camel might reject the DidComm component if it comes with repositories other than central