Speed: Ability to provide required keys in pushTransaction

[n13]

Provide keys instead of looking up required keys

I notice that getRequiredKeys makes multiple calls to the chain - I would like to have a way to provide only required keys in the first place, since whenever I call push, I know exactly which keys I need.

This way I would save myself another 4 or 5 calls and make transactions a lot faster.

The API could accept a list of signing keys.