Minimal Spec Support

[dmarzzz]

mev-boost component of issue: flashbots/mev-boost-relay#738 on the mev boost relay

[dmarzzz]

Minimal spec currently asumes access to /eth/v1/config/spec on the beacon API but mev boost does not currently call any beacon APIs

[metachris]

This is the minimal spec PR for the relay: flashbots/mev-boost-relay#740