Add test to ensure scalar_type.fbs remains in sync across the executorch repo

[lucylq]

🚀 The feature, motivation and pitch

Scalar_type.fbs is copied in a few places across the executorch repo:

extension/flat_tensor/serialize/scalar_type.fbs

devtools/bundled_program/schema/scalar_type.fbs

schema/scalar_type.fbs

Write a test, similar to this one, that keeps them all in sync.

cc @JacobSzwejbka @angelayi

[JacobSzwejbka]

Instead of doing this can we just reuse the same schema?