Cannot use mix of flashbots/mev-rs images in mev_params

[jtraglia]

Right now, with mev_type we can use all of flashbots or all of mev-rs. It would be useful if we could pick and choose each. If for example there's a bug with flashbots builder, we could swap it out with mev-rs builder as a sanity check. Or if, for example, flashbots builder isn't ready yet, we can use mev-rs builder in the meantime.