Implement PayloadBuilder

[frisitano]

Overview

We must implement a PayloadBuilder that can build new payloads. This builder will receive an invocation via the engine API with an argument of PayloadBuilderAttributes as defined in #57. We must extract L1 messages from the PayloadBuilderAttributes and combine with transactions from the transaction pool to build a block.

Considerations

• We must ensure that we implement transaction validity checks such as ensuring that we do not include transactions that have the ability to increase proving disproportionately compared to the transaction fees as this could be a DOS vector.
• We must ensure that L1 messages are included at the top of the block.
• We must ensure that block size limit is upheld.