Add a new BuildTxWith type

During tx construction we want to annotate the scripts into their use
sites, however the final representation of txs puts the scripts
separately. Unfortunately we cannot reverse this transformation so we
end up with this asymmetry between building and viewing a transaction.

This data type helps us deal with this asymmetry, at tx build type it
lets us annotate with another type, but provides nothing when viewing.
The build vs view mode is determined by another type param.

cardano-api/src/Cardano/Api/TxBody.hs

@@ -633,6 +633,22 @@ updateProposalSupportedInEra AllegraEra = Just UpdateProposalInAllegraEra
 updateProposalSupportedInEra MaryEra    = Just UpdateProposalInMaryEra
 
 
+-- ----------------------------------------------------------------------------
+-- Building vs viewing transactions
+--
+
+data BuildTx
+data ViewTx
+
+data BuildTxWith build a where
+
+     ViewTx      ::      BuildTxWith ViewTx  a
+     BuildTxWith :: a -> BuildTxWith BuildTx a
+
+deriving instance Eq   a => Eq   (BuildTxWith build a)
+deriving instance Show a => Show (BuildTxWith build a)
+
+
 -- ----------------------------------------------------------------------------
 -- Transaction output values (era-dependent)
 --