dxp/issues#879: improve mocks

changes

• new runtime system based on 4 variables, layout, input, pagination and results
• adjust and fix minor issues

test

• check the readme update
• update the backend config when needed
• check that the new available commands are usable
• provide feedback about the new mocks system, anything you would prefer to have in a different way

concerns

dxp/issues#899 is still relevant