Add CreateAttachmentStream type

[mkrasnitski]

Addresses #2690 by adding a type which allows lazily creating attachments which only get resolved at request time. The Clone requirement means that we can't store the Reader types directly, which is unfortunate but not too bad.

Draft for now because I don't know how to concisely add support to the create_* builders. Adding support to the edit_* builders was easy by just adding an EditAttachments::add_stream method. However, the create_* builders have three separate methods for adding attachments: add_file, add_files, and files. Duplicating all three to accommodate the streaming variant seems quite noisy.

[GnomedDev]

I don't see how this solves the linked issue. These types still require loading all the files into memory before sending them, instead of streaming them, it just defers the load to the builder execution instead of the builder creation. Implementing proper streaming would probably require dropping down to hyper, but I'm not sure.

[mkrasnitski]

I guess I was thinking about long-lived builders which keep their data around in memory. If we want to use readers directly, we'll have to drop the Clone requirement which would require separating out the attachment methods for builders so that the builders themselves can still remain Clone.

[mkrasnitski]

Closing this for now as it isn't the solution we really want.