Fixed#776
- Add new feature gh-api-client to binstalk-downloader
- Impl new type `binstalk_downloader::remote::{RequestBuilder, Response}`
- Impl `binstalk_downloader::gh_api_client::GhApiClient`, exposed if `cfg(feature = "gh-api-client")` and add e2e and unit tests for it
- Use `binstalk_downloader::gh_api_client::GhApiClient` to speedup `cargo-binstall`
- Add new option `--github-token` to supply the token for GitHub restful API, or read from env variable `GITHUB_TOKEN` if not present.
Signed-off-by: Jiahao XU <Jiahao_XU@outlook.com>
