serve-git: port src-expose serve to hidden sub command
Created by: keegancsmith
This initial commit copy pastes the serve code from the Sourcegraph repo at commit d0d94c90. It's mostly copy paste, but we remove some side-effects we no longer needed to rely on.
I'd like to continue developing this against master and when it is more ready I'll document it in the CLI and other docs.
Part of https://github.com/sourcegraph/sourcegraph/issues/9243 Part of https://github.com/sourcegraph/sourcegraph/issues/12363