aconfig: first iteration of Rust codegen

Add a new `create-rust-lib` command to generate Rust code. The output is
a src/lib.rs file; the build system is assumed to set the generated
crate's name.

For READ_ONLY flags, the generated code returns a hard-coded true or false.

For READ_WRITE flags, the generated code reaches out to DeviceConfig via
the cc_library server_configurable_flags via the
libprofcollect_libflags_rust Rust bindings. The build system is assumed
to add this to the generated crate's dependencies.

Note: libprofcollect_libflags_rust seems generic enough that it should
be moved to an official Rust wrapper for server_configurable_flags. This
is tracked in b/284096062.

Summary of module the built system is assumed to wrap the auto-generated
code in:

  rust_library {
      name: "lib<namespace>_rs",
      crate_name: "<namespace>_rs",
      edition: "2021",
      clippy_lints: "none",
      no_stdlibs: true,
      lints: "none",
      srcs: ["src/lib.rs"],
      rustlibs: [
          "libprofcollect_libflags_rust",
      ],
  }

Also add a set of test input to be used in the unit tests for a more
coherent test strategy. A follow-up CL will migrate the code in
commands.rs, codegen_java.rs and codegen_cpp.rs.

Bug: 279483360
Bug: 283907905
Test: atest aconfig.test
Test: manual: create cache from files in testdata, create rust lib, add to module template above, verify the module builds
Change-Id: I02606aa3686eda921116e33f7e2df8fd1156a7aa