[Auth] Pass custom headers returned from auth webhook/jwt to remote schemas

I want to get all the headers returned from auth webhook/jwt in the remote schema. So that I want have to re-do same auth logic in the remote schema.

### Describe the solution you'd like

Provide a flag e.g. `HASURA_GRAPHQL_AUTH_HOOK_FORWARD_HEADERS` to allow user to enable this feature.

No files, tests, or entry points are identified in the issue. Start by locating the auth webhook/JWT handling and remote schema request forwarding, then determine how the proposed HASURA_GRAPHQL_AUTH_HOOK_FORWARD_HEADERS flag should control forwarding and how completion should be tested.

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