Add mapping from C++ program::verification to Python (#5915)

Summary:
As titled. This enables
`portable_lib._load_for_executorch[_from_buffer]` to accept `Program::Verification` argument.

See added test, now we can do something like:

```
from executorch.extension.pybindings.portable_lib import Verification
module = load_fn(
  exported_program.buffer,
  enable_etdump=False,
  debug_buffer_size=0,
  program_verification=Verification.Minimal,
)
```

Pull Request resolved: https://github.com/pytorch/executorch/pull/5915

Test Plan: See unit test

Reviewed By: dbort

Differential Revision: D63987538

Pulled By: larryliu0820

fbshipit-source-id: b68d8d1149e2d46b90544679707f420179e72b19