pdl: Add support for struct fields

The canonical tests now pass for packets using struct fields: we
support all combinations of scalars and enums in the structs.

The missing field types are padding, custom, and, checksum fields. For
arrays, we still lack support for size modifiers.

Bug: 230475725
Tag: #feature
Test: atest pdl_tests pdl_rust_generator_tests_{le,be}
Change-Id: I92024364796b4c366cbf7460d146136b3763fe7e