R1:3db175f79ddd

Add a "content" attachment for Pastes for Conduit API

Summary: Ref T9964. Builds on D14772. Allows callers to get the raw content of pastes as an attachment.

Test Plan:
- Read docs.
- Executed attachment query.
- Saw raw paste content.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T9964

Differential Revision: https://secure.phabricator.com/D14774