Add API to submit transactions using runtime scale_value::Value's

[jsdw]

A transaction can basically be represented as a pallet name, call name, and a vec of scale_value::Value's representing each argument to the call. (In theory the whole thing could be represented by a single Value, but is there an advantage to that or does it just leave more room for errors?).

Let's add an API to allow transactions of this form to be created and submitted, ideally reusing as much as possible of the existing logic.