rusty-gd: add builder structs

by making them structs, you must name the parameters explicitly (which
is great for readability - also it doesn't matter which order you
specify them as long as you specify them all)

since builders can be constructed for middle structs in the lineage, add
None option for child enums

Bug: 171749953
Tag: #gd-refactor
Test: gd/cert/run --rhost SimpleHalTest
Change-Id: Ic2cfde113ccd6f8475c7514c513ac7b62b3689e3