Discovery: Add `opel` key to the execution layer

[trianglesphere]

We can move V5 bootnodes to the superchain registry and use the ENR text format. It's also possible to use DNS discovery for the EL (a little important b/c reth does not yet support discV5 but is moving in that direction.).

V5 can be cross network. We will want to split EL and CL. Long term we may want to remove testnet bootnodes & just have mainnet bootnodes.

Right now most bootnodes are stored as enodes in our code but should be ENRs.