Add subject field to PrivateMessage

[PitchforkAssistant]

There is currently no way to get the subject line of a private message. The PrivateMessage getters don't include the private message subject.

Being able to read the subject would allow Devvit apps to mimic the behavior of certain old bots that rely on the subject line.

I inspected the protos data passed into the PrivateMessage constructor, that doesn't include the message subject either, so this would likely require changes to the non-public protos package.