Allow custom fetch on mutations (small PR)

[scottBowles]

Currently, queries accept a custom fetch (mainly for SvelteKit's SSR fetch) but mutations use the browser's fetch, since they will always have access to it. This PR will allow the developer to pass a custom fetch to both.

Why?

For those of us using external apis and needing to add an Authorization header to the request. Like the SvelteKit RealWorld example app, I store a jwt in a cookie, receive the cookie in the handle hook, and put the jwt in the session. In order to hit the external api directly with gQuery rather than routing everything through a SvelteKit endpoint, it makes sense to retrieve the token from the session store whenever I need to hit the api. Given access to the fetch function, I can easily wrap fetch and add the token to the Authorization header. But gQuery's current implementation doesn't allow for this on mutations since I have no access to the fetch function being used.

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[stolinski]

Good call but I think I have something different in mind to solve this. Will update soon.