pdl: Extract generate_data_struct helper

This helper will be used to generate the common code for both packets
and structs.

Tag: #feature
Bug: 228306436
Test: atest pdl_tests pdl_rust_generator_tests_{le,be}
(cherry picked from https://android-review.googlesource.com/q/commit:e441c3308a95385c9c023b781fe457243b759ecb)
Merged-In: Ib193838a5d7b0786ede72ee53ebe580b6d847b8f
Change-Id: Ib193838a5d7b0786ede72ee53ebe580b6d847b8f