[land-bot] Add buildbot support for requests to land diff patches
See D7424 for rationale behind landing Differential diffs.
This patch adds support for diffs to the /land endpoint of the buildbot.
The funtionality is not yet available, so hitting this endpoint in production
will either fail or dry-run until the workflow is completed.
Test Plan: ninja check-buildbot
Reviewers: #bitcoin_abc, deadalnix
Reviewed By: #bitcoin_abc, deadalnix
Differential Revision: https://reviews.bitcoinabc.org/D7425