feat: allow providing custom fetch implementation

[jacobwgillespie]

What kind of change does this PR introduce?

This PR adds the ability to provide a custom fetch implementation as an option - this is most useful for environments where cross-fetch is not supported, for instance Cloudflare Workers or Vercel Edge Functions.

This is related to supabase/supabase-js#154, and I believe a similar option would need to be added to the other libraries using cross-fetch before being finally added to supabase-js.

What is the current behavior?

At the moment, cross-fetch is always used in all environments.

What is the new behavior?

There is a new option fetch that can be provided to override cross-fetch with an alternative library (or the native fetch):

import { SupabaseStorageClient } from '@supabase/storage-js'

const url = '...'
const headers = {}
const fetch = ...

const storage = new SupabaseStorageClient(url, headers, fetch)

Additional context

Wrapped library PRs:

• feat: allow providing custom fetch implementation #24 (this PR)
• feat: allow providing custom fetch implementation postgrest-js#222
• feat: allow providing custom fetch implementation auth-js#168

supabase-js PR:

• feat: allow providing custom fetch implementation supabase-js#297

[kiwicopple]

🎉 This PR is included in version 1.5.0 🎉

The release is available on:

• npm package (@latest dist-tag)
• GitHub release

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