Adding source for flutter discord tree-status-bot. (flutter#3552)

GCP cloud function zip for the tree status bot which publishes tree status to discord channel.

Bug:flutter/flutter#144063

*Replace this paragraph with a description of what this PR is changing or adding, and why. Consider including before/after screenshots.*

*List which issues are fixed by this PR. You must list at least one issue.*

*If you had to change anything in the [flutter/tests] repo, include a link to the migration guide as per the [breaking change policy].*