Add a HttpIO / HttpFileSystem

Add HttpIO (and a related HttpFileSystem and HttpsFileSystem), which can download files from a particular http:// or https:// URL. HttpIO cannot upload / write to files, though, because there's no standardized way to write to files using HTTP.

Sample usage:

        (
            p
            | ReadFromText("https://raw.githubusercontent.com/apache/beam/5ff5313f0913ec81d31ad306400ad30c0a928b34/NOTICE")
            | WriteToText("output.txt", shard_name_template="", num_shards=0)
        )