Implement DoH server for testing

This is the initial implementation of DoH tests. Similar to how
DnsTlsFrontend works, the code in rust is a proxy which forwards
DoH queries to a backend cleartext nameserver. The code in C++ is
a wrapper for tests to communicate with Rust DohFrontend.

Bug: 155855709
Bug: 181642979
Test: m libdoh_frontend_ffi
Change-Id: If1d7ab1bdd242a9b4be4b1f72c453c6a86245dbe