Remove setters from PsbtV2.PSBT_GLOBAL_INPUT_COUNT and PsbtV2.PSBT_GLOBAL_OUTPUT_COUNT

[Shadouts]

The psbtv2 key PSBT_GLOBAL_INPUT_COUNT and PSBT_GLOBAL_OUTPUT_COUNT represent the number of inputs or outputs on the psbt. On the PsbtV2 class, these values have setters, however, because setters are public, they might cause these values to disagree with the real number of inputs or outputs tracked in the psbt.

The two values must remain in sync with the number of inputs or outputs being tracked in the map.

I think the easiest way to do this is to remove the setters and only allow the add and delete input/output methods to modify the value of these fields. However, this is a backward incompatible change for those that might require the existence of the setters.