Refactor method.h to hide forward-declared flatbuffer types

Summary:
We'd like to support newer versions of `flatbuffers`, but the `Vector` template type has changed in a way that would require `#ifdef`-ing the forward declaracitons.

Instead, just hide the types completely. `parse_values()` already touches `Method` fields, so there's not a lot of benefit to passing in values from other fields.

With this change, I was able to successfully build against flatbuffers v23.5.26 in https://github.com/pytorch/executorch/pull/39

Reviewed By: cccclai

Differential Revision: D48138016

fbshipit-source-id: dad7f1b27fe90d0d9092acd7e4960aba42ed0d9d