refactor(unspents): add, use Dimensions.SingleInputs
refactor(unspents): add, use Dimensions.SingleInputs

* Add `Dimensions.SingleInputs` (complement SingleOutputs)
* Add `Dimensions.fromScriptType(ScriptType, params)` helper
* Change `assumeUnsigned` type to `Dimensions`

This consolidates a lot of code where we instantiated single inputs
in very similar ways.

Issue: BG-40758

BitGo/BitGoJS,BitGo/BitGoJS,BitGo/BitGoJS,BitGo/BitGoJS