Add an example and README for background tasks

[sebsto]

Add code example for Lambda Background Tasls

Motivation:

Partially addresses #376

Modifications:

Add a directory Examples/BackgroundTasks with code sample and a README

I also slightly modified Lambda+Codable.swift to provide default JSONEncoder() and JSONDecoder() to not require user to create their own if they don't need to.

Result:

Developers know how to use Lambda background tasks

[sebsto]

This code requires to merge #417 to work
It will compile but no logger debug statement are printed without #417