[v3] Add credentials to RequestInit

Is your feature request related to a problem? Please describe.

node-fetch’s request type does not include credentials: https://github.com/node-fetch/node-fetch/blob/cbd4d895767e33491219e40013dd8daa5c7ac024/%40types/index.d.ts#L56

This is a field provided by the web platform: https://developer.mozilla.org/en-US/docs/Web/API/Request/credentials

Describe the solution you’d like

Add credentials field

Describe alternatives you’ve considered

I could work around it, but would prefer not to have to

Additional context

This is used in the upcoming SvelteKit: https://github.com/sveltejs/kit/blob/95c47bcdb884194359475968b1e0eeb342820611/packages/kit/src/runtime/server/page/load_node.js#L129

SvelteKit’s predecessor Sapper did something similar as well: https://github.com/sveltejs/sapper/blob/339c417b24e8429d3adc9c9f196bf159a5fce874/runtime/src/server/middleware/get_page_handler.ts#L137

[jimmywarting]

the only thing we could support is the omit the other two (same-site and include) would have the same meaning on backend

So there is a tiny usecase for it… but since we don’t have a cookie jar either… then it have even less usefulness

[benmccann]

We don’t want things to work on the server-side and then break on the client-side because it’d be very difficult for users to see different behavior depending on whether they did a server-side or client-side navigation to the page. All we were requesting here was to change the type so that we can check if (opts.credentials !== 'omit') ourselves without running into an issue of credentials not being defined on the type. I wasn’t suggesting to change the node-fetch behavior

Let’s leave the SvelteKit discussion in that repository to avoid confusing this issue