feat(net): accept discv5 ENR bootnodes via CLI

[MysticRyuujin]

Description

--bootnodes and --trusted-peers reject enr: records:

error: invalid value 'enr:-KO4QO5...' for '--bootnodes <BOOTNODES>': Failed to parse url: no host specified

This makes it impossible to bootstrap from a discovery bootnode that publishes an ENR (e.g. ethpandaops/bootnodoor on kurtosis devnets), even though the discv5 stack supports ENR boot nodes internally.

This teaches TrustedPeer::from_str to decode enr: records via the existing TryFrom<&Enr> for NodeRecord; the parsed record flows through the existing bootnode wiring into both discv4 and discv5.

• A discovery-only ENR (no tcp key) falls back to the udp port for the RLPx dial guess, same as Discv5::try_into_reachable — otherwise a tcp-less ENR in --trusted-peers would be redialed at port 0 forever.
• Without the secp256k1 feature the parser now returns a clear error instead of the misleading URL error (library builds only; the feature is always on in the reth binary).
• CLI help text and generated docs updated.

Verified on a kurtosis devnet: reth bootstraps and meshes with nethermind and besu through a bootnodoor discv5 ENR (together with #26449).

Possible follow-up: the ENR is down-converted to an unsigned NodeRecord (same treatment as enode URLs), so the signed BootNode::Enr/add_enr seeding path remains unreachable from the CLI.