Add jarmode support to the loader code

Update the `Launcher` class to allow a packaged jar to be  launched in
a different mode. The launcher now checks for a `jarmode` property and
attempts to find a `JarMode` implementation using the standard
`spring.factories` mechanism.

Closes gh-19848

spring-boot-project/spring-boot-tools/spring-boot-loader/src/test/resources/META-INF/spring.factories

@@ -0,0 +1,2 @@
+org.springframework.boot.loader.jarmode.JarMode=\
+org.springframework.boot.loader.jarmode.TestJarMode