Add a configuration option to skip ClearlyDefined

Sometimes, we can't reach ClearlyDefined and we end up hanging while we
wait. By specifying "skip" as the ClearlyDefined URL skips the provider.

core/src/main/java/org/eclipse/dash/licenses/context/LicenseToolModule.java

@@ -62,7 +62,10 @@ public int getWeight() {
 				return 100;
 			}
 		});
-		licenseDataProviders.addBinding().to(ClearlyDefinedSupport.class);
+
+		if (!"skip".equals(settings.getClearlyDefinedDefinitionsUrl())) {
+			licenseDataProviders.addBinding().to(ClearlyDefinedSupport.class);
+		}
 
 		bind(LicenseSupport.class).toInstance(new LicenseSupport());
 		bind(GitLabSupport.class).toInstance(new GitLabSupport());