pdl: Add structs to Rust canonical tests

We can parse the definitions fine (we parse them like a packet), but
we cannot yet handle struct fields.

Test: atest pdl_tests pdl_rust_generator_tests_{le,be}
(cherry picked from https://android-review.googlesource.com/q/commit:e7fb30313fbd12d91087dc3ed610b923805ff381)
Merged-In: Idd14f43ad0cf32ebeeaa532b15b36259e0808728
Change-Id: Idd14f43ad0cf32ebeeaa532b15b36259e0808728