P2P discovery

[protolambda]

Implements:

• ENR entry (chain ID and version)
• Background process to do random DHT walk, filter nodes, and add to the peerstore
• Background process to select random, not connected, peers and connect to them
• Discovery test: A is the bootnode, B and C find each other via A

[codecov-commenter]

Codecov Report

Merging #439 (ebb8c2c) into main (b37e658) will increase coverage by 0.71%. The diff coverage is 64.74%.

@@            Coverage Diff             @@
##             main     #439      +/-   ##
==========================================
+ Coverage   51.55%   52.27%   +0.71%     
==========================================
  Files          70       70              
  Lines        7582     7855     +273     
==========================================
+ Hits         3909     4106     +197     
- Misses       3151     3197      +46     
- Partials      522      552      +30     

Flag	Coverage Δ
unittests	52.27% <64.74%> (+0.71%)	:arrow_up:

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
op-node/flags/p2p_flags.go	100.00% <ø> (ø)
op-node/node/node.go	61.47% <0.00%> (-0.77%)	:arrow_down:
op-node/p2p/config.go	7.77% <0.00%> (-0.06%)	:arrow_down:
op-node/p2p/rpc_server.go	61.65% <ø> (ø)
op-node/p2p/prepared.go	26.08% <8.33%> (-23.92%)	:arrow_down:
op-node/p2p/node.go	58.69% <52.38%> (+4.02%)	:arrow_up:
op-node/p2p/discovery.go	70.32% <70.00%> (+58.20%)	:arrow_up:
op-proposer/txmgr/txmgr.go	93.78% <0.00%> (-1.87%)	:arrow_down:
op-node/rollup/driver/state.go	72.99% <0.00%> (-0.33%)	:arrow_down:
... and 1 more

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[trianglesphere]

Concurrency diagram

[protolambda]

• Fixed imports / magic numbers
• Opened an issue for possible discovery improvements: #445

I will prioritize testnet + scaling first, but then plan to revisit peer-scoring, and can then address the discovery process once we've a better scoring / peer-tracking system in place.