add context variables and stage variables to API NG context

[bentsku]

Motivation

We need a separate context, representing the $context variables used to render VTL templates, and which is also passed in the payload to AWS_PROXY integrations.

More informations about $context here. https://docs.aws.amazon.com/apigateway/latest/developerguide/api-gateway-mapping-template-reference.html#context-variable-reference

It is a bit of duplicated information from the original RestApiInvocationContext object and the InvocationRequest, but they serve different purposes.

If we need to update different contexts at the same time, then maybe we could simplify. But we learned that creating those context when needed, collecting informations from different sources, can be pretty hard.

The InvocationRequest goal is simply to represent the incoming request "as is", with no transformation.

The idea would be to go from InvocationRequest, get the Resource. Use the original InvocationRequest, the Resource,the ContextVariables and StageVariables to construct the IntegrationRequest and send it down. The InvocationRequest will not be mutated. The ContextVariables will be mutated and populated along the way of the different handlers.

Changes

• add the ContextVariables and populate it
• add the not yet used LoggingContextVariables which can add more data to the logging
• add IntegrationRequest for future work in integration request handler
• add the stage variables and fetch them from the stage
• add small tests to validate we're populate the contexts properly

TODO

What's left to do:

• [x] add unit tests