Add manifest entry to indicate that a jar has been built for native image

[scottfrederick]

When a Spring Boot jar has been built for use in a native image, an entry named Spring-Boot-Native-Processed with a value of true should be added to the META-INF/MANIFEST.MF file. This indicates that the jar has been AOT processed, the GraalVM reachability metadata is enabled, and reachability metadata has been copied to the jar. Tooling such as Cloud Native Buildpacks can use this manifest attribute to make decisions about whether to package the application as a native executable.

This attribute will be absent from the manifest if the jar has not been prepared for use in a native image.

[scottfrederick]

Closed with 23ae91b