Use custom fetch(er) for web client

[arfath-linklet]

By default the web client uses the global fetch. The idea is to configure the client with a custom fetch(er) that may decorate the fetch function with additional functionality. One example we encountered -

• Using mTLS: Cloudflare Workers can use mTLS with added bindings, the binding exposes a fetch(er) with mTLS pre-configured https://developers.cloudflare.com/workers/runtime-apis/bindings/mtls/

[slvrtrn]

Can you please create an issue for that?

[arfath-linklet]

#315