Add jni-config generation

This commit adds support generating graalvm `jni-config.json` file.

Configuration format for jni/reflection in graalvm is documented
to be exactly same so we can re-use facilities for reflection hints
which should be relatively clean for a user as also graalvm uses same
classes for both jni/reflection.

Closes gh-29007

spring-core/src/test/java/org/springframework/aot/nativex/FileNativeConfigurationWriterTests.java

@@ -49,6 +49,7 @@ import static org.assertj.core.api.Assertions.assertThat;
  * Tests for {@link FileNativeConfigurationWriter}.
  *
  * @author Sebastien Deleuze
+ * @author Janne Valkealahti
  */
 public class FileNativeConfigurationWriterTests {
 
@@ -149,6 +150,23 @@ public class FileNativeConfigurationWriterTests {
 				]""", "reflect-config.json");
 	}
 
+	@Test
+	void jniConfig() throws IOException, JSONException {
+		// same format as reflection so just test basic file generation
+		FileNativeConfigurationWriter generator = new FileNativeConfigurationWriter(tempDir);
+		RuntimeHints hints = new RuntimeHints();
+		ReflectionHints jniHints = hints.jni();
+		jniHints.registerType(StringDecoder.class, builder -> builder.onReachableType(String.class));
+		generator.write(hints);
+		assertEquals("""
+				[
+					{
+						"name": "org.springframework.core.codec.StringDecoder",
+						"condition": { "typeReachable": "java.lang.String" }
+					}
+				]""", "jni-config.json");
+	}
+
 	@Test
 	void resourceConfig() throws IOException, JSONException {
 		FileNativeConfigurationWriter generator = new FileNativeConfigurationWriter(tempDir);