Consider adding a hooks for a custom build step

[lance]

The Google Cloud Functions runtime has capabilities for custom build steps by providing a hook via a gcp-build task in package.json. According to the documentation,

When this script is executed, the dependencies in the dependencies and devDependencies fields of your package.json file are available. After executing your custom build step, Cloud Functions removes and regenerates the node_modules folder by only installing the production dependencies declared in the dependencies field of your package.json file.

This might be a nice feature to add to our buildpacks. I'm not sure how exactly this would work for go and quarkus. @lkingland @matejvasek any thoughts?