Allow to build several handlers in the same lambda

[nfroidure]

Currently we build only one lambda and export it as the default export :

whook/packages/whook-aws-lambda/src/index.ts

Line 363 in 9c7249f

export default handler;

It sometimes makes sense to group several lambda function into the same zip we it shares most of their services to limit the number of simultaneous db connections of even cold starts.

The current build system is very modular and could be adapted easily to do so.