Lambda specific custom handlers

[ari-luokkala]

I would like to have different handlers for different lambdas (default, api etc.).

With inputs different handlers could be assigned to different lambdas.

This gives flexibility on custom handlers.

e.g.

inputs:
handler:
defaultLambda: ${custom.handler.${stage}}
apiLambda: "index.handler"
imageLambda: "index.handler"