TransactionRouter keeps track of a given transactions state and is used to update command requests with the appropriate transaction-related metadata (e.g. txnNumber). It currently provides a few helper functions for this, most notably attachTxnFieldsIfNeeded, appendFieldsForStartTransaction, and appendFieldsForContinueTransaction. However, manipulating BSON directly is error prone (it has very few type-safety guarantees, appending duplicate keys can be easy) and difficult to read/maintain, as well as not being performant (mutating often involves scanning the BSON and reallocating).

Instead, we should update these functions to operate over IDL-generated commands or their GenericArguments structs. This will allow us to write more type safe, performant, and easier-to-maintain code.