Add a way to pad transactions' size if needed by Simplicity cost metric

Simplicity transactions have an additional "cost" associated with the computational resources consumed by verification of the results of executing the contract.

This cost is paid for by ensuring that the transaction has a sufficiently high weight (using conventional weight calculations) to cause the associated fee to be adequate according to the cost formula. Currently, most typical Simplicity programs inherently produce large enough transactions to satisfy this requirement, so there hasn't been a need to artificially increase the size of their transactions in order to get them accepted by Liquid nodes. However, this might not always be the case for more computationally-intensive contracts in the future, e.g. those that use loops or that call expensive jets repeatedly for some reason.

So, we could use a standardized way to pad transactions to make them deliberately larger when this turns out to be required by the cost formula.

@apoelstra suggested that the appropriate place to specify and implement the basis for this is here in rust-elements.

該 issue 提到了 rust-elements，但沒有指定檔案或測試。首先定位交易 weight 與序列化的進入點，以及現有的 Simplicity 成本處理；然後確定 padding 的依據和所需的涵蓋範圍。完成的標準是實作一個經過測試的標準化機制，用於將交易 padding 到所需的 weight。