Auth Webhook should support multiple roles X-HASURA-ROLES

It's normal that the user can have many roles. Eg: user = john can be contributor & admin

When using Auth Webhook, as of https://github.com/hasura/graphql-engine/issues/1420#issuecomment-455849733, developers should resolve multiple user roles to a particular role.

To do that, developers must somehow resolve GraphQL operation name (can't be trusted), variables, and query (pass as string from webhooks) to the GraphQL data -> the user's role -> return to Hasura. I think the point of Hasura is to reduce friction for developers to adopt GraphQL, and it defeats the point of using Hasura.

The straightforward way is for developers to return all user roles using X-HASURA-ROLES and lets Hasura handle the rest like JWT Auth.

[Auth Webhook's POST request](https://hasura.io/docs/latest/graphql/core/auth/authentication/webhook.html#post-request)
```
{
"headers": {
"header-key1": "header-value1",
"header-key2": "header-value2"
},
"request": {
"variables": {
"a": 1
},
"operationName": "UserQuery",
"query": "query UserQuery($a: Int) {\n users(where: {id: {_eq: $a}}){\n id\n }\n}\n"
}
}
```