Use Dr.CI results to classify flaky failures in trymerge (pytorch#110054)

After pytorch/test-infra#4589, we can now query Dr.CI to get the list of flaky failures there.  This change queries Dr.CI API endpoint and check if the failure is a flaky one using `is_flaky` function.

Because the change is relatively large, I'm breaking it down to several smaller PRs in this order:

* [x] This PR queries Dr.CI and adds `is_flaky` check
* [ ] Clean up the flaky rules logic because it has already been implemented on Dr. CI
* [ ] Clean up the broken trunk logic for the same reason

### Testing

* Create a new `drci_mocks.json` file to catch the JSON response from Dr.CI API endpoint. The API requires `DRCI_BOT_KEY`.
*  `pytest -v test_trymerge.py`

Pull Request resolved: pytorch#110054
Approved by: https://github.com/clee2000

Author: huydhn