Provider should expose a way to get the RelayRequest directly and be able to submit it later

[irux]

Is your feature request related to a problem? Please describe.
I want to be able to send the transaction over the backend. I don't want the frontend to submit the transaction directly.

Describe the solution you'd like
The provider doesn't allow to sign the transaction out of the box. We don't have any easy way to get a RelayRequest object to be sent over the network and then be submitted by other entity.