rusty-gd: integrate rusty packets into existing code

still gotta wait for parsing to land for this to officially work, but
this allows us to play with the ergonomics independently based on what
compiles..

next CLs will start to improve the ergos.

some ergos improved this CL.

Bug: 171749953
Tag: #gd-refactor
Test: gd/cert/run --rhost SimpleHalTest
Change-Id: I5eadd7619c5d392c2dd03331c5d604d9a52a1687