new AOT BundledProgram class to store ExecutorchProgram instead of Program (#1213)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/1213

This diff updates bundled program aot to replace Program with ExecutorchProgram-like object (`ExecutorchProgram`,`MultiMethodExecutorchProgram` or `ExecutorchProgramManager`) to unblock sdk usage.

To achieve that, we create a new class, also called `BundledProgram`, but under `bundled_program/core`, to make bunded program contains `ExecutorchProgram` information while still only serialize the `Program` object in the given `ExecutorchProgram` when serializing `BundledProgram` to flattbuffer. The new `BundledProgram` object will be the only `BundledProgram` users interact with, and in the future we will add more apis to make it more user friendly. We also update relative apis like `create_bundled_program`, `serialize_from_bundled_program_to_flatbuffer`, etc, to meet the new class.

This diff only focuses on BundledProgram info update to unblock SDK asap. Other parts, like the class structure, documentation, serialization apis, etc, still needs to be updated.

Reviewed By: tarun292

Differential Revision: D51126519

fbshipit-source-id: 3518be18934d6b74f10cab1732b6951338941d69