Add support for custom HTTP headers when generating

[giddingsra0]

See title.

An example is when interacting with the Shopify GraphQL API, auth is done via an API token sent in a custom header like X-Shopify-Access-Token=************. Being able to pass in this header or key/value pair would be beneficial. Currently, I cannot get the schema from Shopify with the GraphQLinq.Scaffolding tool.