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