Allow to define spring.cloud.function.definition property value as a manifest entry and a system property for GcfJarLauncher

[PierrickPuimeanChieze]

Currently, defining spring.cloud.function.definition as an environment variable is the only way to specify the name of the function to run for GcfJarLauncher. But you can specify the class containing the function to run as a system property (MAIN_CLASS) or a manifest entry (either Main-Class or Start-Class).

For consistency, I think it could be good to be able to define the name of the function to run either as a system property (spring.cloud.function.definition) or as a manifest entry (Function-Definition come to mind, to be coherent with the property and envvar name, but any other key could do the job).