Add defaultHeader config automatically passed to Apollo Client

[eric-burel]

Goal

This helps setting up API keys for instance, that you may want to be sent in all your graphql queries.
Eg to setup API keys #54

Steps

• Add a configuration item in next.config.js where we can add default headers (check the progress of File based configuration management #62 before doing this)
• Load the default headers based on this configuration in apollo client
• Advanced: demo a custom header computation with Apollo client setContext and middleware system