ra-data-hasura: lazy header

[heyrict]

Currently, updating cookie or localStorage in authProvider will not update the headers in hasuraDataProvider in ra-data-hasura.

Since header contents can have authentication information stored in cookies or localStorage sometimes, may be we can add a callable as the headers argument aside from a constant object. Using httpClient similar to simpleRestProvider described here as the second argument is also acceptable.

[mihazs]

This is the same issue of #2619 currently isn't possible to integrate any type of authentication that uses the headers,

[shahidhk]

Fixed in 0a64ef9