Make JUnit5 TestcontainersExtension public (#5285)

This allows more flexible and programmatic usage of the `TestcontainersExtension`

Fixes #2045

Author: hmatt1

modules/junit-jupiter/src/main/java/org/testcontainers/junit/jupiter/TestcontainersExtension.java

@@ -32,7 +32,7 @@
 import java.util.stream.Collectors;
 import java.util.stream.Stream;
 
-class TestcontainersExtension
+public class TestcontainersExtension
     implements BeforeEachCallback, BeforeAllCallback, AfterEachCallback, AfterAllCallback, ExecutionCondition {
 
     private static final Namespace NAMESPACE = Namespace.create(TestcontainersExtension.class);