Fetcher and Headers should be mutable per request.

[ayazemre]

Hello,

Thank you for the great library. I hit couple roadblocks. When I create client on client side it works great. When I need to write tests I need to add custom headers, this is not allowed after client is created. Also for fullstack frameworks like SvelteKit, we have custom fetcher on the server side which works as event bus. I need to reinstantiate the client on server side, which seems unnececssary. I think it would be helpful to have client created with given data but when you do a query or mutation I should be able to individually set those parameters per request for edge cases. I can edit the library myself but need to fork it then. What do you think about this request?

[linear]

GEN-37 Fetcher and Headers should be mutable per request.

[remorses]

You can pass a function for headers to change them

https://genql.dev/docs/usage/create-the-client#e670ccc1623a47bfa6a482a297e03f47

[remorses]

Instantiating a new client is a cheap operation