Add fetch instance option

[chanced]

This fixes #403 by adding fetch as an option to GrpcWebTransportOptions so that users of SvelteKit gain various benefits from providing a fetch instance (docs).

My prettier config mangled the formatting. Is there anyway you could please share your prettier config so that I can align it?

edit:

This was the easiest path forward. Arguably, it would be slightly better performance wise to add fetch as an optional parameter to the returned functions. However, doing so would require a bit more changes, including the generators.

I'm willing to make those changes if that's the path you'd prefer.

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[timostamm]

Apologies for the late reply, @chanced. It looks like an option to provide fetch in the transport is the best way forward.
Having to create a new transport instance for every server side render is not ideal, but I don't think it matters much in the real world.

The pattern could be to define a function for creating clients with the desired transport at a single place in the app:

export function createClient<T extends ServiceType>(service: T, fetch?: FetchFn): PromiseClient<T> {
  const t = createConnectTransport({baseUrl: "/", fetch});
  return createPromiseClient(service, t);
}

And than use it in the SvelteKit load functions:

export async function load({ fetch, params }) {
  const c = createClient(SomeService, fetch);
  return await c.someMethod({id: params.id});
}

Some initial testing by @paul-sachs looked promising, but there are issues with binary data and we did not look into server-streaming yet. Supporting SSR via a custom fetch sounds great, but we need more investigation and also integration tests to make sure it works properly.

[timostamm]

Superseded by #618.

Thank you for the contribution, @chanced!