[llvm] [Debuginfod] Add HTTP Server to Debuginfod library.

This provides a minimal HTTP server interface and an implementation wrapping [[ https://github.com/yhirose/cpp-httplib | cpp-httplib ]] in the Debuginfod library. If the Curl HTTP client is available (D112753) the server is tested by pinging it with the client.

Reviewed By: dblaikie

Differential Revision: https://reviews.llvm.org/D114415