[RFC] Deploy any CDK construct

[CorentinDoue]

linked to #182

This is a draft to discuss supporting generic CDK constructs and Lift constructs. Here is how it can be used so far:

service: app

provider:
    name: aws

constructs:
    myCdkConstruct:
        type: customCdk
        cdkConstructPath: ./cdkConstruct.ts

    myLiftConstruct:
        type: customLift
        cdkConstructPath: ./liftConstruct.ts

plugins:
    - serverless-lift