Build support for Rust via Cargo

ChromeOS is missing support for mixing C++ and Rust in GN so we compile
Rust in a separate step using Cargo. Currently, only the direct code
portions were copied to the Cargo.toml and tests are still missing. The
final output from the Rust build is the libbt_shim_ffi staticlib.

Some BUILD.gn rules for rust libraries were also added (to try to make
it work with an experimental Rust toolchain in GN) but was abandonded
because dependencies couldn't be built. Leaving this in since it will be
useful when GN supports building Rust as well on ChromeOS.

Bug: 176847256
Tag: #floss
Test: atest --host bluetooth_test_gd
Change-Id: I4d1ab599309b153b2304cb87c4526bc476dc2637