pdl: Add support for fixed fields

Both scalars and enums are supported and tested.

Test: atest pdl_tests pdl_rust_generator_tests_{le,be}
(cherry picked from https://android-review.googlesource.com/q/commit:ac5c368e847fef0e7c02e0ac01ea8023d824a9d2)
Merged-In: I2e3252f3a4756dc5339517d5287f5b7b7a28f285
Change-Id: I2e3252f3a4756dc5339517d5287f5b7b7a28f285