Summary:
Context: I'm trying to add support for sending packager progress events to the client that is downloading the bundle over HTTP multipart response.
The idea is for the client to send `Accept: multipart/mixed` header, and if present the server will stream progress events to the client. This will ensure the change is backwards-compatible - the clients who don't know about progress events won't receive them.
In the future we can use this approach to download RAM bundle modules in one request.
Reviewed By: davidaurelio
Differential Revision: D3926984
fbshipit-source-id: 39a6e38e40a79f7a2f2cf40765a0655fb13b7918