feature-request: Weighted relays

### Diversify relay usage

There is a community driven pressure to diversify the relays, mostly because having a single relay control a big percentage of the block production gives that relay powers to censor or delay some transactions.

Right now if a user wants to avoid using a relayer due to those concerns it has to avoid including this relay when setting up mev-boost. The problem with this approach is that the user is now deprived from any offers from the relay, even when those offers are on-off opportunities that doesn't affect regular block production.

Adding weights to relays would allow users to give priority to relays who align with their principles while still having access to other relays in case they have a good offer. It allows expressing intents like "use relayer Y unless X pays double".

### Better risk management

Weighted relays would also allow users to include "less trusted" relays but only when these relays have substantial better offers, expressing things like "I don't fully trust relay X but if it pays ten times more I'm willing to take the chances".