added support for cache keys from header mappings, (e.g. method.request.body)

[mgyarmathy]

HI @DianaIonita!

Similar to PR https://github.com/DianaIonita/serverless-api-gateway-caching/pull/65, I needed a way to configure API Gateway to cache responses from my GraphQL Lambda function using method.request.body as a cache key.

In order to prevent breaking changes, my solution was to branch off of the existing code if the cacheKeyParameter includes a value property, which indicates the cache key is from a mapped HTTP header. This new functionality also opens up the possibility of using other values like context.VARIABLE_NAME in mapping expressions as well.

I have updated the README and added additional tests to document this new functionality.

With the following configuration, I was able to achieve my desired configuration for a sample /graphql endpoint:

cats-graphql:
    handler: graphql/handler.handle
    events:
      - http:
          path: /graphql
          method: get
          caching:
            enabled: true
            cacheKeyParameters:
              - name: request.querystring.query
      - http:
          path: /graphql
          method: post
          integration: lambda
          caching:
            enabled: true
            cacheKeyParameters:
              - name: integration.request.header.bodyCacheHeader
                value: method.request.body

GET

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