Create cgmanifest.json and third_party/cgmanifest.json and fill it out

Notes:

• External libraries should be moved to third_party where possible
• Google Benchmark and libcurl were not added since they do not appear to be checked into the repo. They are mentioned in https://github.com/microsoft/cpp_client_telemetry/blob/main/docs/List-of-OSS-Components.md