Add a new field to OutboundP2PMessage

Summary:
Some boilerplate for writing and reading the field from D12742.

https://linear.app/comm/issue/ENG-8729/automatic-retries-for-non-composable-dms

Depends on D12742

Test Plan: Tested later in the stack by checking if the message with this field can be saved and read.

Reviewers: kamil, marcin

Reviewed By: kamil

Subscribers: ashoat

Differential Revision: https://phab.comm.dev/D12743