Union accessors in C# should use generic type for the table

When accessing a union field, we should return the same object type
as was given to the method, i.e. the parameter should have a generic
type for any Table-derived type. This way, we do not need to make
superfluous casts (which also reduce type safety) like

var myUnionType = (MyUnionType)buff.GetUnionField(new MyUnionType());

when we can do just
var myUnionType = buff.GetUnionField(new MyUnionType());

Change-Id: Idac1b638e46cc50b1f2dc19f10741481202b1515