[ci] Create comment bot to highlight CI runtime changes

[driazati]

It's difficult to know whether a PR will affect CI runtime without doing a purposeful investigation or hitting timeouts. Similar to the tests_bot (https://github.com/apache/tvm/blob/main/.github/workflows/tests_bot.yml) we could have a bot that just comments the runtime change of the last Jenkins build vs the median of the prior N builds from main

cc @Mousius @areusch @gigiblender