ConstructionPreprocessRequest : ConstructionPreprocessRequest is passed to the /construction/preprocess endpoint so that a Rosetta implementation can determine which metadata it needs to request for construction. Metadata provided in this object should NEVER be a product of live data (i.e. the caller must follow some network-specific data fetching strategy outside of the Construction API to populate required Metadata). If live data is required for construction, it MUST be fetched in the call to /construction/metadata.