Add missing constraints to TransactionBuilderStep

[euonymos]

Some constraints are missing in the TransactionBuilderStep:

• signatures
• validity intervals
• reference an output (?)
• not be valid (?)

[klntsky]

I suggest not implementing any constraints that don't automate anything but rather just attach data to the tx body. We did that in the past and it was just overengineering. Lenses or even pure tx body updates are just fine

[euonymos]

From the user's perspective, they are the same. Additionally, it's expected since there is no comment on why some bits are missing.