Allowing external checks on Github PR

Leiqing Cai

I'd like to propose allowing additional external checks on Github PR. Specifically, I'm looking to a test to check for Facebook integration.

Currently, Travis CI is run on each PR (shown in the screenshot below).
However, this is not enough since there are active repos depending upon the master of prestodb/presto.
Allowing adding external integration checks can help identify breakages earlier on.

Here are the rules that I propose for external checks:
- Anyone can add external checks upon TSC approval.
- Test owner should make sure that the test is not flaky.
- Test failures does not block merging. They only serves alerting purpose.

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