feat: add blockrelay.privileged-builders

[zimbatm]

Privileged builders are a list of public keys of builders from which bids will be accepted first, even if the bid is lower. If no bids are received from the priority builders, bids from other builders will be accepted.

This is useful when you have a special contract with specific relays and you would like to have their blocks used instead of other relays. While still having other relays as a fallback.

I wanted to submit this to you early to see if this is something you would be interested in having.

[mcdee]

I think it makes sense to merge this pretty much as-is, we can look at a more generic way of handling this later on if needs be.

One item: please could you fix the commit to the docs/configuration.md so that it only includes the required changes rather than the reformat? Thanks.

[zimbatm]

Great, thanks for reviewing our work @mcdee

[MrKoberman]

Made a small change to pass the deepsource check.