1. Make coverage script accept an env var that controls where the generated report is written to. Use this in generate_coverage_report.
2. Rename CoverageGetter to OSSFuzzCoverage
3. Make affected_fuzz_targets get coverage object from clusterfuzz_deployment.
4. Add stubbed FilesystemCoverage class that will be used by non-OSS-Fuzz users.
5. Make ClusterFuzzLite able to return coverage object by downloading old coverage report and returning a FilesystemCoverage object.
6. Add stub to ClusterFuzzLite for uploading coverage.
Related: #6054
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