After #1918 (review) (to avoid merge conflicts) we can change DataStore::get_transaction_inputs to return a TransactionInputs. This should make that interface nicer and avoid the tuple return type.

This requires removing InputNotes from TransactionInputs. Due to the changes in the above PR, we may want to introduce another private TransactionInputsWithNotes (better name) that contains a TransactionInputs and InputNotes. This would be returned by prepare_transaction_inputs.

This will also require moving the input note related checks from TransactionInputs to that new type.

For context, this was previously discussed in #401.