Support javaagent instrumentation with loader

Update spring-boot-loader to allow `-javaagent` instrumentation when
running from executable jars.

Prior to this commit the `Launcher` skipped the application classloader
and instead used the system classloader as a parent. This was to ensure
that locally packaged classes were always loaded by the classloader
that had access to nested jars.  Unfortunately when using the
`-javaagent` option, it is the application classloader that is modified.

The `Launcher` class now uses the application classloader as parent
and `LaunchedURLClassLoader` has been updated to always search local
URLs before delegating to the parent. This is very similar to the way
that most application servers handle the loading of war files.

Issue: #56232870