Investigate building in the server build directory

ns_server is the only project generating build artifacts in the same directory as the code. This has resulted in additional cleanup scripts/cmake to clean directories in the ns_server repository. We should investigate moving the build artifacts to the server build directory to make cleanup easier and common to all server projects. Rebar3 looks to provide us with good options to choose directories for build artifacts - https://rebar3.org/docs/configuration/configuration/#directories.