[Feature] Add option to provide custom fetch implementation

[jahands]

I want to use capnweb with VPC which has it's own fetch function that I would need to pass into capnweb so that it uses that instead of global fetch

[jahands]

I did confirm that it works over VPC by temporarily overriding globalThis.fetch (which is a terrible idea):

try {
	globalThis.fetch = (req: any, init: any) => c.env.VPC.fetch(req, init)

	// do RPC stuff here
} finally {
	globalThis.fetch = currentFetch
}

[kentonv]

On Workers you can actually do:

newHttpBatchRpcSession(new Request(url, {fetcher: c.env.VPC}))

As long as VPC is actually a Fetcher runtime type.

But this relies on a very old, undocumented runtime API, and it only works with the built-in Fetcher type.

It would make sense to let people provide a custom fetch function in options, I agree.