fix(transaction-builder): Require send_iota amount param

[DaughterOfMars]

Description

The amount param for send_iota was optional, but this meant that not providing it would send the entire gas coin, which is probably undesirable. This PR makes it required.

[thibault-martinez]

I wouldn't say it was undesirable nor dangerous, seems pretty expected to me that no amount means the whole thing. But I'm fine changing it anyway.

[DaughterOfMars]

I wouldn't say it was undesirable nor dangerous, seems pretty expected to me that no amount means the whole thing. But I'm fine changing it anyway.

I agree with this in the case of send_coins where you specify the coins you want to send. But here the gas may be chosen for you and I think, even though it should be obvious that providing no amount would have SOME effect, it's not clear in this case what effect that would be.