Default disallow example non-DID VC issuer

[clehner]

VC Test Suite requires being able to issue a verifiable credential out of a test vector credential whose issuer property is https://example.edu/issuers/14.

ssi checks verification relationship when issuing verifiable credentials. But this checking expects a DID URI as issuer, which resolves to a DID document, that contains verification material. In the case of a HTTP issuer URI (URL), something analogous should probably be done, but we don't have support for that yet. So for this example.edu HTTP issuer, the check is simply skipped, and for all HTTP issuers, an error is raised. This behavior addresses the current requirements. However, it might not be advisable to allow the check to be skipped in this case, outside the vc-test-suite test driver that requires it. Typically, handling of an example URL like this might be gated by a #[cfg(test)] attribute. However, the vc-test-suite test driver is run as an executable, so I don't think that the test configuration option can be used here. Instead, this PR adds a new feature for it, example-http-issuer. This feature is then disabled by default but enabled for use in the ssi-vc-test package.