Local server api descriptors contain urls that work on Mac and Linux

Docker for Mac does not allow host networking and thus will not allow SDK harnesses to access runner services via `localhost`. Instead, a special DNS name is used to refer to the host machine: docker.for.mac.host.internal. (Note that this value sometimes changes between Docker releases).

We should attempt to detect the host operating system and return different API descriptors based on this.

See https://github.com/bsidhom/beam/commit/3adaeb0d33dc26f0910c1f8af2821cce4ee0b965 for how this might be done.