feat: improve configuration options

see comment from @achingbrain at https://github.com/ipfs/helia-http-gateway/pull/63#discussion_r1514661428:

> I wonder if we couldn't just merge the USE_BITSWAP and USE_LIBP2P vars to just be use @helia/http or use helia?
>
> Like, USE_HTTP_GATEWAYS_ONLY or something to select @helia/http.

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I think the original intent was to surface metrics for the following:

1. helia with libp2p & bitswap + trustless gateways + delegated routing
2. helia with libp2p and delegated routing only (no bitswap)
3. helia via trustless gateways only.

so we can see where fetching is the least performant and target improvements there. ipns://probelab.io/websites/#websites-http-comparison-ttfb-p90 has some explainers.