Similar to what protobufs does with its `Message` interface, introduce here such interface and create a generic `GetRootAs` method to deserialize a flatbuffer.
When creating a CreateXxx(...) method for a simple table type,
enum-type fields with a non-zero default must have an explicit
cast for the respective argument default value, because in C#,
there is an implicit cast from int to an enum only for 0.
Also, added an example of such type into the example monster_test
type, so that we test this feature.
gonzaloserrano