Use the HTTP Last-Modified http header as the mtime value for ADD cmd when present

Closes #8331

Signed-off-by: Doug Davis <dug@us.ibm.com>
This needs some simplifying and clarifying. How about:

If the remote file being retrieved has an HTTP Last-Modified header, the timestamp from that header will be used to set `mtime` on the destination file. Then, like any other file processed during an `ADD`, `mtime` will be used to determine whether or not the file has changed and the cache should be updated.