fix: respect routers config in helia factory

[2color]

What's in this PR

• take the passed routers config in createHelia into acount

This PR allows overriding the routers configuration when customising Helia. Useful when you want to exclusively fetch directly from peers without relying on recursive gateways.

Relevant context

In @helia/http we already do this:

helia/packages/http/src/index.ts

Lines 79 to 82 in 332580c

	routers: init.routers ?? [
	delegatedHTTPRouting('https://delegated-ipfs.dev'),
	httpGatewayRouting()
	]

[SgtPooki]

lgtm!