Allow Bootnodes to be specified under nested domains in the chain spec

[wpank]

Right now bootnodes are individually defined in the chain spec, like this:

  "bootNodes": [
    "/dns/p2p.cc3-0.kusama.network/tcp/30100/p2p/12D3KooWDgtynm4S9M3m6ZZhXYu2RrWKdvkCSScc25xKDVSg1Sjd",
    "/dns/p2p.cc3-1.kusama.network/tcp/30100/p2p/12D3KooWNpGriWPmf621Lza9UWU9eLLBdCFaErf6d4HSK7Bcqnv4",
    "/dns/p2p.cc3-2.kusama.network/tcp/30100/p2p/12D3KooWLmLiB4AenmN2g2mHbhNXbUcNiGi99sAkSk1kAQedp8uE",
    "/dns/p2p.cc3-3.kusama.network/tcp/30100/p2p/12D3KooWEGHw84b4hfvXEfyq4XWEmWCbRGuHMHQMpby4BAtZ4xJf",
    "/dns/p2p.cc3-4.kusama.network/tcp/30100/p2p/12D3KooWF9KDPRMN8WpeyXhEeURZGP8Dmo7go1tDqi7hTYpxV9uW",
    "/dns/p2p.cc3-5.kusama.network/tcp/30100/p2p/12D3KooWDiwMeqzvgWNreS9sV1HW3pZv1PA7QGA7HUCo7FzN5gcA",
    "/dns/kusama-bootnode-0.paritytech.net/tcp/30333/p2p/12D3KooWSueCPH3puP2PcvqPJdNaDNF3jMZjtJtDiSy35pWrbt5h",
    "/dns/kusama-bootnode-0.paritytech.net/tcp/30334/ws/p2p/12D3KooWSueCPH3puP2PcvqPJdNaDNF3jMZjtJtDiSy35pWrbt5h",
    "/dns/kusama-bootnode-1.paritytech.net/tcp/30333/p2p/12D3KooWQKqane1SqWJNWMQkbia9qiMWXkcHtAdfW5eVF8hbwEDw"
  ]

This leaves some inflexibility in adding and removing nodes. Each addition or deletion requires a new client release, which can take a decent amount of time before the changes get rolled out to node operators.

Instead, it would be nice to be able to be able to provide a domain list, such as [https://bootnodes.parity.io, https://bootnodes.polkadot.network], where each domain can list a IPFS CID -> QmY7Yh4UquoXHLPFo2XbhXkhBvFoPwmQUSa92pxnxjQuPU. Fetching this content of this CID would then yield the previous list of bootnode multiaddrs. This approach makes it easier to dynamically add and remove bootnodes to networks at any point (irregardless of client release cycles).

CC @bkchr

[tomaka]

There already is a libp2p feature where you'd pass /dnsaddr/bootnodes.polkadot.network. It's not implemented in rust-libp2p yet: libp2p/rust-libp2p#1450
https://github.com/multiformats/multiaddr/blob/master/protocols/DNSADDR.md

[tomaka]

Implemented in libp2p: libp2p/rust-libp2p#1931
Remains to update libp2p in Substrate.

[romanb]

Remains to update libp2p in Substrate.

I'm preparing a PR.

[tomaka]

Done in #8420