Extend the uri::Fetcher::Plugin interface to include a "fetchSize"

In order to replace the mesos-fetcher binary with the uri::Fetcher, each plugin must be able to determine/estimate the size of a download. This is used by the Fetcher cache when it creates cache entries and such.

The logic for each of the four {{Fetcher::Plugin}}s can be taken and refactored from the existing fetcher.
https://github.com/apache/mesos/blob/653eca74f1080f5f55cd5092423506163e65d402/src/slave/containerizer/fetcher.cpp#L267