Have an actual bootstrap node for Substrate-based chains

[nazar-pc]

Right now bootstrap nodes for Substrate-based chains are full proper blockchain nodes, which is actually neither necessary nor desired.

It would be much better to have a lightweight bootstrap node implementation that only serves the purpose of network bootstrapping and peer discovery, but doesn't actually run the blockchain operations as such.

Especially in light of issues like #6433 and #6434 current design causes substantial network issues and significantly increases infrastructure costs in parts of the world where connectivity is limited and/or bandwidth is more expensive (we have challenges with bootstrap nodes in China where servers with wide bandwidth channel are really expensive comparing to servers in Europe or America).

Ideally it'd be a small separate application that can be run separately and use very little bandwidth comparing to full consensus node.

[bkchr]

The node basically just needs to run Kademlia.

[nazar-pc]

Pretty much