[bs-55] Add strategy for setting system exit code

* Added ExitCodeGenerator and SpringApplication.exit
convenience method
* User can add bean of type ExitCodeGenerator or supply
one in the call to exit()

[Fixes #48475971]

spring-bootstrap-samples/spring-bootstrap-simple-sample/src/test/java/org/springframework/bootstrap/sample/simple/SimpleBootstrapApplicationTests.java

@@ -45,7 +45,7 @@ public class SimpleBootstrapApplicationTests {
 	}
 
 	@Test
-	public void testCommandLoneOverrides() throws Exception {
+	public void testCommandLineOverrides() throws Exception {
 		SimpleBootstrapApplication.main(new String[] { "--name=Gordon" });
 		String output = getOutput();
 		assertTrue("Wrong output: " + output, output.contains("Hello Gordon"));