Add metaspan (IBP) boot node

[dcolley]

What does it do?

Adds a new boot node to the chain-spec

What important points should reviewers know?

This has been tested internally:

./bin/moonbeam --tmp --chain ./chain-spec/moonbeam.json --bootnodes "/dns/boot.metaspan.io/tcp/51143/p2p/12D3KooWQNKbTaWXMAFV24Ytgjy93ZXLVSM3sJSbf8LNKqu3yrUM" --no-hardware-benchmarks --relay-chain-rpc-urls=ws://192.168.10.16:9944
# OK
./bin/moonbeam --tmp --chain ./chain-spec/moonbeam.json --bootnodes "/dns/boot.metaspan.io/tcp/51146/wss/p2p/12D3KooWQNKbTaWXMAFV24Ytgjy93ZXLVSM3sJSbf8LNKqu3yrUM" --no-hardware-benchmarks --relay-chain-rpc-urls=ws://192.168.10.16:9944
# OK

Is there something left for follow-up PRs?

no

What alternative implementations were considered?

n/a

Are there relevant PRs or issues in other repositories (Substrate, Polkadot, Frontier, Cumulus)?

no

What value does it bring to the blockchain users?

enhanced decentralisation