Similar to what protobufs does with its `Message` interface, introduce here such interface and create a generic `GetRootAs` method to deserialize a flatbuffer.
Also fixed most codegenerators using the wrong namespace when multiple
namespace were used in a file, with some files not being generated.
Change-Id: Ib42969221239d7244e431cbd667ef69200fc415f
Tested: on Linux.
Bug: 29338474