Abstraction for Builder Transactions at the end of the block

Add trait to abstract any transactions the builder would want to add at the end of the block e.g. payout transaction, block signature etc.

Before block building there can be some methods for validation and gas reservation e.g. https://github.com/flashbots/rbuilder/issues/296. After block building the trait can specify the transactions to include, akin to a bundle at the end of the block from the builder.