Add support for setting headers

[ulisses-cruz]

Reason

Sometimes it is necessary to set the headers in a GraphQL request, for example to send a token.

How would it work

# headers: {
#   "token": "e969e767-8e92-44aa-9f03-6f3c49b43c26"
# }
query GetUserData {
  getUserData {
    id
    name
    email
  }
}