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 #154 and depends on the following PRs in the wrapped client libraries:

• feat: allow providing custom fetch implementation postgrest-js#222
• feat: allow providing custom fetch implementation auth-js#168
• feat: allow providing custom fetch implementation storage-js#24

All three of those PRs would need to be merged before this PR builds correctly / functions.

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 { createClient } from '@supabase/supabase-js'

// Provide a custom `fetch` implementation as an option
const supabase = createClient('https://xyzcompany.supabase.co', 'public-anon-key', { fetch: fetch })

Additional context

Dependent PRs:

• feat: allow providing custom fetch implementation postgrest-js#222
• feat: allow providing custom fetch implementation auth-js#168
• feat: allow providing custom fetch implementation storage-js#24

[kiwicopple]

🔥 🔥 🔥 🔥 🔥

This is a very impressive series of PR's @jacobwgillespie - let me run some tests then merge if everything looks good