Remote execution implementation

[tmckayus]

This change replaces the solve_lp_remote and solve_mip_remote stubs with real routines that use an embedded grpc client to communicate with a remote cuopt server.

There are two documents included, GRPC_ARCHITECTURE.md and SERVER_ARCHITECTURE.md that act as developer guides.

The server is built by build.sh and is called cuopt_grpc_server.

Remote execution is still enabled via env vars CUOPT_REMOTE_HOST and CUOPT_REMOTE_PORT

To try this feature, checkout the branch/PR and do the following:

$ build.sh
$ cuopt_grpc_server // defaults are fine, run with -h for help
$ CUOPT_REMOTE_HOST=localhost CUOPT_REMOTE_PORT=8765 cuopt_cli myproblem.mps

All cuopt APIs will pick up remote execution if the env vars are set. So cuopt_cli, the C API, the Python API, all will solve problems on the server if the env vars are set and the server is running. Just use cuopt tools and APIs as you normally would for a local solve.