[fidl][sdk][cpp] Make table setters chainable

This:

MyTable t;
t.set_x(1);
t.set_y(2);
MyFunction(std::move(t));

Can now be reduce to:

MyFunction(MyTable().set_x(1).set_y(2))

Test: Table.ChainSetters; all existing tests using tables also aren't
failing.

FIDL-615

Change-Id: Ieeffa04417e92df7c0bb01d3b461dd968ba33852