Add third party crates

We manage all third party crates in the https://github.com/vivoblueos/external repository. All crates are transformed into GN’s BUILD.gn files via gnrt which is initially created by the Chromium project.

Add dependency in //external/Cargo.toml

To add a new third party crate, for example unwinding, add following code in //external/Cargo.toml

[dependencies.unwinding]
version = "=0.2.4"
default-features = false
features = [
    "unwinder"
]

Then run ninja -C <out> run_gnrt. After the above two steps, developers can add //external/vendor/unwinding-0.2.4:unwinding to the deps field in the target defintions in BUILD.gn.