Add factory for easier creating of dependencies

onionwrapper-core/src/main/java/org/briarproject/onionwrapper/CircumventionProviderFactory.java

+package org.briarproject.onionwrapper;
+
+import org.briarproject.nullsafety.NotNullByDefault;
+
+@NotNullByDefault
+public class CircumventionProviderFactory {
+
+    public static CircumventionProvider createCircumventionProvider() {
+        return new CircumventionProviderImpl();
+    }
+
+}