Adds greptile.json file for specifying triggers for Greptile on PRs (#4008)

Adds greptile.json file for specifying triggers for Greptile on PRs
This should avoid Greptile from getting triggered on every commit for a
PR.
It should still be able to get triggered manually by @ the bot.
Signed-off-by: Ammar <43151630+AmmarYAhmed@users.noreply.github.com>

greptile.json

+{
+  "triggerOnUpdates": false
+}