Support batched submissions in DAP

[akoshelev]

There is currently interest from the PPA group to use DAP as the main protocol to aggregate measurement results. However, there is a significant difference in how reports are submitted to the MPC: Rather than coming from end user devices, they are collected by an intermediary and later submitted to the Leader in bulk. The PPA group concluded that network bandwidth savings are worth having a DAP extension that enables bulk uploads.

In w3c/ppa#36, @cjpatton expressed support to have this functionality implemented in DAP, so this issue is open to see if there is appetite from this group to support it.

The formal proposal to change DAP can be found here (aside from section 4.1 that is I believe is addressed in dap-13)

Previous work

This issue has been discussed in #64 and #78. This group concluded that the complexity of supporting batch submissions (particularly handling partial failures) was not worth the network savings obtained from grouping request metadata in one header.

[cjpatton]

My main recollection from previous discussions: we concluded that if batched uploading could be handled at the HTTP level, than we would perhaps better serve higher level applications to force them to do just that.

If the reports your batch-uploading have fields in common, then you can in theory save a little bandwidth:

1. the task ID (32 bytes per upload)
2. any public extensions that have identical payloads (savings vary based on the contents of extensions)
3. the taskprov advertisement, if you're using that extension and using it to provision tasks (savings vary, perhaps a 100 or so bytes per report)

How "big" might report extensions get for PPA? Would the payloads be the same across all or at least many of the reports?

Regardless of how much we might save, I think if sending a very large HTTP request instead of many small HTTP requests is convenient for PPA, then we should support it.