1. Move most of the generic stuff into BasePlatformConfig.
2. Make a GCB platform config that sets WORKSPACE and PROJECT_SRC_PATH so users don't have to.
3. Make a skeleton prow platform_config for Mitchel to fill out.
4. Make users explicitly specify the CFL_PLATFORM so we can pick CI environments
Implement filestore based on github actions' artifacts feature.
This uses the github api and the github actions API.
Also fix imports in github_actions_toolkit library that were broken by move to third_party directory.
jonathanmetzman