Support progressing reading of files in multipart uploads in v3

[xsawyerx]

v3 supports multipart uploads which use the multipart/related envelope.

Due to having to construct this manually (since HTTP::Request doesn't support it), we are not reading the file content iteratively using the helper method _content_sub(). Arguably, another _content_sub() could be written[1] that iteratively generates not just the file content but the multipart/related envelope prior to it. This would allow progressively providing the file content instead of loading the entire file into memory.

This isn't necessarily the best use of time at the moment, because the limits of multipart uploads are 5M anyway. Once resumable uploads are available, those allow any size (including 5M and below) in a strictly progressive manner out of the box, so they could also be used (though would probably be somewhat more verbose).

[1] This can also be accomplished by adding a hook to the existing _content_sub().