Allow customization of GraphQL fetch

The GraphQL fetch should allow for more custumization:

- Currently the fetch call is fixed to query `elastic50`, feels weird to have to use elastic50 as my query name even if I'm using 7.2
- enforces the usage of a `host` parameter(which I dont use as the graphql proxy takes care of the host + credentials, therefore resulting in an error for me)
- Uses content type `application/graphql` while lots of implementations like Apollo seem to prefer `application/json`

- ReactiveBase could take a transformRequest parameter, which seems to be ignored in case `graphQLUrl` is used.