pdl: Implement specialize functionality

This mirrors the “inheritance” implemented by the old
bluetooth_packetgen compiler.

Tag: #feature
Bug: 228306436
Test: atest pdl_tests pdl_rust_generator_tests_{le,be}
(cherry picked from https://android-review.googlesource.com/q/commit:381a8c070d53a382a2d2d15e1a4c17cb70a591b9)
Merged-In: I8f66bfce60e95b13446242c32c02098c5416e304
Change-Id: I8f66bfce60e95b13446242c32c02098c5416e304