make the underlying HttpClient configurable

Current GraphQLet gets a Bad Request 400 when invoke api of Node.js + graphql-yoga.

By trace the headers, I found that I can reproduce the scenario when using curl by adding header Upgrade: h2c. Not sure if this is a bug of the server express.js which graphql-yoga used. Actually express.js does not support http/2 yet, see https://expressjs.github.io/statusboard/

So, we need add some compatibility to GraphQLet for configaring the HttpClient.

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add a new constructor to allow set HttpClient by user self.

public GraphQLet(HttpClient client, String serviceUrl)