Allow layer creation of user built modules

[AmanL04]

I am new to serverless and from the jump, I noticed that layers using the plugin "serverless-layers" cannot include user-defined modules (common code).

Please add support to allow layer creation from folders (which can also include requirements.txt)

[m0uneer]

Inspecting the source code, I found this undocumented option copyBeforeInstall

  serverless-layers:
    customInstallationCommand: npm run myCustomCommand # If you use NodeJS, but you can set the command that suits your case
    copyBeforeInstall:
      - package-lock.json  # Again, if you use NodeJS so please check the repo for default files needed for every package manager
      - path/to/custom/files # This file will be part of the layers

In addition, the customInstallationCommand option can give the flexibility needed to do anything with copyBeforeInstall files set.