aconfig: Rust codegen 2nd iteration

Introduce rust codegen 2nd iteration with unit test support. The design
is described in detail in the design doc (go/aconfig_rust_codegen). The general idea is to generate
different code with the same signature for production and test build
targets, just like java/c/cpp codegen.

We will have a FlagProvider struct that has flag methods implementation.
This flag provider instance can then be used in injection pattern. In
additon, we also generate top level functions that wraps around flag
provider call so it can be used in static function style.

Things to be decided later:

should we just generate one set of code, and use cfg! as compile
time marco to compile the right code for the right targets.

Bug: b/279483360
Test: atest aconfig.test
Change-Id: Ic75cedbd0d27b5242014c3ac7fc80692d2ab4589