Support for pass HTTP Headers in Graphiql

[RohitUNC]

Feature description

Currently, we can only pass query variables in /graphiql, however, at times we need to pass header parameters like x-api-key or other custom headers to some apis which is tough to test out using graphiql and we've to resort to restful clients like postman to test those apis.
If this can be enabled in graphiql (similar to this: https://developers.cloudflare.com/analytics/graphql-api/getting-started/authentication/graphql-client-headers) that would be great feature here.

[frisi]

refs #262