Prevent Tomcat URL "reflective access" warnings

Update the jar `Handler` class to support a non-reflective fallback
mechanism when possible. The updated code attempts to capture a regular
jar URL before our handler is installed. It can then use that URL as
context when creating the a fallback URL. The JDK jar `Handler` will
be copied from the context URL to the fallback URL.

Without this commit, resolving new Tomcat URLs of the form
`jar:war:file:...` would result in an ugly "Illegal reflective access"
warning.

Fixes gh-18631

spring-boot-project/spring-boot-tools/spring-boot-loader/src/test/java/org/springframework/boot/loader/jar/HandlerTests.java

@@ -163,6 +163,7 @@ class HandlerTests {
 		URLConnection jdkConnection = new URL(null, "jar:file:" + testJar.toURI().toURL() + "!/nested.jar!/",
 				this.handler).openConnection();
 		assertThat(jdkConnection).isNotInstanceOf(JarURLConnection.class);
+		assertThat(jdkConnection.getClass().getName()).endsWith(".JarURLConnection");
 	}
 
 	@Test