feat(rbuilder): Work-in-progress: supprt in-process reth-rbuilder

[ryanschneider]

Still testing this, but it seems to work!

$ go run main.go --use-bin-path --watch-payloads --use-in-process-rbuilder-config ~/rust/rbuilder/config-playground.toml

...
Starting reth version v1.0.6-dev
Starting lighthouse version v5.3.0
Services started:
==================
- reth (authrpc: 8551, http: 8545, rpc: 30303)
- beacon_node (http: 3500)
- validator ()
- mev-boost-relay (http: 5555)

All services started, press Ctrl+C to stop
Block Proposed: Slot: 2, Builder: 0xa1885d66bef164889a2e35845c3b626545d7b0e513efe335e97c3a45e534013fa3bc38c3b7e6143695aecc4872ac52c4, Block: 2
...

Some cleanup tasks:

• log starting reth-rbuilder to make it more clear which binary its running
• currently only works w/ --use-bin-path, either add reth-rbuilder artifacts or log an explicit error

[ferranbt]

This is not required anymore since after #29 we can run two EL nodes on the playground.