Refactor the uri::Fetcher as a binary.

By refactoring the uri::Fetcher as a binary, the fetcher can be used independently. Not only mesos, but also new fetcher plugin testing, mesos cli and many other new components in the future can re-use the binary to fetch any URI with different schemes. Ideally, after this change, mesos cli is able to re-use the uri::Fetcher binary to introduce new image pulling commands, e.g., `mesos fetch -i <path>`.