cache the names of functions returning JWT and avoid extra query on each rpc call

src/PostgREST/Types.hs

@@ -13,6 +13,7 @@ data DbStructure = DbStructure {
 , dbColumns     :: [Column]
 , dbRelations   :: [Relation]
 , dbPrimaryKeys :: [PrimaryKey]
+, dbProcsReturningJWT :: [Text]
 } deriving (Show, Eq)
 
 type Schema = Text