pdl: Add support for payloads and constraints

This CL adds support for payload fields and child packets.

The data struct for each packet is now marked `pub`: this is because
it can be mentioned in the enums we build to hold child data.

Tag: #feature
Bug: 233340327
Bug: 233340326
Test: atest pdl_tests pdl_rust_generator_tests_{le,be}
Change-Id: I8284f3a7f5b4a7e5c6ae12077a8466492d74cea0