refactor(snack-sdk): replace fetch ponyfill with global fetch

[krystofwoldrich]

Why

I was running into the following error when using the Snack SDK in CF Workers.

I've tracked down the issue to an old node-fetch.

Error on server: TypeError: Cannot read properties of null (reading 'has')
    at processHeader (node-internal:internal_http_outgoing:904:39)
    at ClientRequest._storeHeader (node-internal:internal_http_outgoing:209:21)
    at ClientRequest._implicitHeader (node-internal:internal_http_client:414:14)
    at ClientRequest.#write (node-internal:internal_http_outgoing:823:18)
    at ClientRequest.write (node-internal:internal_http_outgoing:607:32)
    at ClientRequest.write (node-internal:internal_http_client:382:22)
    at writeToStream (snack-sdk.js:10347:10)
    at snack-sdk.js:10688:5
    at new Promise (<anonymous>)
    at fetch2 (snack-sdk.js:10493:10)

How

This PR removes the current ponyfill and defaults to global fetch impl of the runtime.

Since all modern browser have a solid fetch implementation, I think we could use the global for web. Likely we could do the same for Expo Apps.

For modern node ponyfill would also not be needed. But for other envs for example CF Workers, we need the ability to change the fetch function. So I exposed a public function setSnackSDKFetch(fn).

Test Plan

[byCedric]

We do need to fix CI before merging, but it looks good overall.