Add support for runtime fetching of context fiels

[PascalSprenger]

Hello, I am absolutely unsure if this is a bug or if I am using it incorrectly.

I would like to create verifiable credentials via didkit-cli. If I use an official context as a source, everything works. Here is my working examples from the tutorial:

			String vcData = String.format(‘’
										            {
					    ‘@context’: 
					    ‘https://www.w3.org/2018/credentials/v1’
					    ‘id’: ‘urn:uuid:12345678-abcd-1234-ef00-123456789abc’,
					    ‘type’: [‘VerifiableCredential’],
					    ‘issuer’: ‘%s’,
					    ‘issuanceDate’: ‘%s’,
					    ‘credentialSubject’: {
					    	‘id’: ‘did:example:abcdef’
					    }
					}‘’", issuerDID, DateTimeFormatter.ISO_INSTANT.format(Instant.now()),claimsJson.toString());

However, if I now try to host the identical context (copy/paste) from https://www.w3.org/2018/credentials/v1 myself via Xampp local, I always get the following error:

Error: thread ‘main’ panicked at C:\Users\xxx\.cargo\registry\src\index.crates.io-6f17d22bba15001f\didkit-cli-0.3.2\src\credential.rs:80:14:
Error: called `Result::unwrap()` on an `Err` value: LDP(ToRdfError(Expand(ContextProcessing(Meta(ContextLoadingFailed(LoadingDocumentFailed(UnknownContext(‘http://localhost/vc2.jsonld’)))), Span { start: 42, end: 71 })))))

Do you have any idea what this could be? I have now tried several different schemas and always get this error.

If I pass a custom context directly via:

					    {
					    ‘dateOfBirth’: ‘http://localhost/definitions/birthDate.html’ 
					    }

so without hosting it, it seems to work.