Support gateway override via IPFS_GATEWAY ENV

[lidel]

Current list of trustless gateways is hardcoded in

ipfs-chromium/library/src/ipfs_client/gateways.cc

Lines 68 to 89 in 0e8d229

	std::vector<std::pair<std::string, int>> ipfs::Gateways::DefaultGateways() {
	return {{"http://localhost:8080/"s, 906},
	{"https://ipfs.io/"s, 902},
	{"https://gateway.ipfs.io/"s, 901},
	{"https://dweb.link/"s, 896},
	{"https://jcsl.hopto.org/"s, 894},
	{"https://ipfs.joaoleitao.org/"s, 858},
	{"https://ipfs.jpu.jp/"s, 328},
	{"https://jorropo.net/"s, 310},
	{"https://gateway.pinata.cloud/"s, 293},
	{"https://ipfs.runfission.com/"s, 256},
	{"https://nftstorage.link/"s, 195},
	{"https://w3s.link/"s, 135},
	{"https://ipfs.scalaproject.io/"s, 69},
	{"https://ipfs.soul-network.com/"s, 63},
	{"https://ipfs.fleek.co/"s, 61},
	{"https://permaweb.eu.org/"s, 51},
	{"https://storry.tv/"s, 41},
	{"https://hardbin.com/"s, 31},
	{"https://ipfs-gateway.cloud/"s, 20},
	{"https://ipfs.storry.tv/"s, 9},
	{"https://ipfs.anonymize.com/"s, 0}};

To make it easier to test against own gateway running on a different port, or dofferent public ones, we could check if IPFS_GATEWAY is present in ENV, and if so, use its value as whitespace-separated list of gateways to use instead of the hardcoded one.

[John-LittleBearLabs]

I wonder if that env name makes it obvious that

• it's plural
• it's instead-of and not in-addition-to
  I'm guessing you chose it for compatibility with other tool(s)?

[lidel]

Yes, it is based on loose convention from ipfs/kubo#8847 and curl/curl#8805,
where we went with IPFS_GATEWAY as most of the time a user will set this to their preferred (local/LAN) gateway, so when set, plural option will rarely be used.
Not set in stone, but would keep it the same, hoping to standardize this at some point (in ipfs/specs#280 or separate IPIP).

[John-LittleBearLabs]

Merged into main under unrelated PR (sorry)