Bip54: add sequence, timestamp (and version) to GBT

In a recent discussion on the Bitcoin Development Mailing List, an inquiry was made about the term "policy surface" and its expansion within the context of Bitcoin development. The question aimed to understand why adding new fields to a proposal, which ostensibly makes implementation easier and safer, is considered an expansion of the policy surface rather than simply a beneficial adjustment. Furthermore, there was curiosity regarding the rationale behind the assertion that these new fields "belong" in a different part of the proposal or discussion, implying that their inclusion might not be as straightforward or beneficial as initially thought. This dialogue reflects an ongoing examination of how modifications to proposals are categorized and their implications for the broader framework and governance of Bitcoin's development process.