[builder API] Enable builder API usage on a per-validator basis

[realbigsean]

Description

Allow enabling or disabling use of the builder API on a per-validator basis. We could store information about whether the builder API should be used for a validator in the validator definitions file. This would give us all the functionality described by the config file in this issue: flashbots/mev-boost#154 event though we don't directly support that format right now.

The changes required would be very similar to those made in this PR for gas limit. I may end up just adding these changes there in order to avoid conflicts #3342