Custom header use to authentication with JWT

Hi,

I'm trying to use a custom header to say Hasura to get the JWT from there, not only the Authorization or Cookie header. Why don't have support for custom headers to access JWT and give more flexibility?

`Fatal Error:- Environment variable HASURA_GRAPHQL_JWT_SECRET: Error in $.header: expected 'type' is 'Authorization' or 'Cookie'`

Start with the HASURA_GRAPHQL_JWT_SECRET configuration and the reported validation error, then trace where the header type is restricted to Authorization or Cookie. Done means a documented custom header can be configured as the JWT source and authentication works through it.

Written by the indexing model from the issue text.