Conditional custom authorizer depending on the stage

This is a Feature Proposal

Description

The use case: You want to add a basic auth (using an authorizer) to access the staging environment but not for the production.
If the authorizer bloc would have an enabled attribute (like the pluging: serverless-domain-manager) we could do something like this:

custom:
  stage: ${opt:stage}
  staging_auth: true
  prod_auth: false 
...
     authorizer:
            enabled: ${custom:stage}_auth
            name: authorizerFunc
            ...

The proposal concerns the custom stage setting and the authorizer block, with authorizerFunc as the referenced entry point. Start by tracing how stage variables and authorizer configuration are parsed; done means the authorizer can be enabled or disabled per stage, with both settings covered by tests.