Spring Add option to disable Last-Modified handling when serving static resources

See spring-io/start.spring.io#519 for the original issue.

When packaging static files, applications may choose to override the file metadata for build reproducibility or caching reasons. In this case, the build is overriding the file modification date to ensure that the hash of the container image layer doesn't change if the content didn't change.

Spring Framework serves static resources and uses that information for the Last-Modified HTTP response header. In the case of the start.spring.io service, the file changed but browsers are reusing the cached version because the modified date didn't change.

We should at least provide a configuration option to disable this behavior if the developers want to.