add 'validationSlot' to transaction builders

[dragospe]

The previous version hard-coded the slot to 1 during ledger rules validation.

It still defaults to 1L in the TxBuilder so as not to break compatibility, but it should probably be updated.

[nau]

I'm actually considering removing validation from transaction building.
It's a separate responsibility and it in no way dependant on transaction building or its context.
It's done in the end and it can easily be extracted and handled separately.
Well, it relies on dummy signatures, but those easy to reconstruct via ctx.expectedSigners.

And we already have the Emulator for transaction validation in case you need it.

@dragospe, what do you think?