feat(network): discv5 discovery

[dancoombs]

#248

Proposed Changes

• Implement a very basic discovery behavior that uses discv5 to discover peers
• A lot of this code will be moved around when we implement the peer manager behavior (or combine this behavior with peer management, haven't decided yet).

[codecov]

Codecov Report

Merging #463 (6c48301) into danc/p2p-rpc (0eb3059) will increase coverage by 0.35%.
The diff coverage is 86.71%.

Files	Coverage Δ
crates/network/src/behaviour.rs	100.00% <ø> (ø)
crates/network/src/discovery/enr.rs	85.71% <100.00%> (ø)
crates/network/src/error.rs	0.00% <ø> (ø)
bin/tools/src/bin/network.rs	0.00% <0.00%> (ø)
crates/network/src/network.rs	79.04% <93.10%> (+1.49%)	⬆️
crates/network/src/discovery/behaviour.rs	85.56% <85.56%> (ø)

Flag	Coverage Δ
unit-tests	56.06% <86.71%> (+0.35%)	⬆️

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

Components	Coverage Δ
rundler binary	0.00% <0.00%> (ø)
builder	45.04% <ø> (ø)
dev	0.00% <ø> (ø)
pool	59.54% <ø> (ø)
provider	1.33% <ø> (ø)
rpc	18.26% <ø> (ø)
sim	82.61% <ø> (ø)
tasks	∅ <ø> (∅)
types	93.56% <ø> (ø)
utils	7.20% <ø> (ø)
network	89.12% <87.40%> (+0.57%)	⬆️