* Works for both CMake and Bazel
* Instrument xds_end2end_test with protos from envoy-api
* Fixed the Bazel repo naming with re2
* Refactor the build metadata compute algorithm
    * Now, in one pass, it will generate all the build metadata needed within
    this script, including the transitive deps, the sources/headers expanded
    without intermediate dependencies.
* Temporarily remove the removal of third_party dependencies
* Spin-off changes to sub PRs
* Add a fetching mechanism for external proto libs
* Accommodate Windows' CMake behavior difference
* Allow CMakefile to fetch external library
    * The URL is parsed from grpc_deps
    * Allow multiple download URLs for extra stability