Add emr cluster link

[anaynayak]

Add an operator link to EMR cluster on AWS Console.

Notes:

1. Followed a similar structure to other operators like DataprocClusterLink
2. Noticed that the tests were a mix of pytest +unittest. Haven't moved the rest of the tests to pytest. Can attempt to do this if it helps.
3. Wasn't sure what all tests to run.

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[boring-cyborg]

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[anaynayak]

Rebased against latest main. I believe the build for the earlier one had failed due to conflicting dependencies (unrelated to this change)

[boring-cyborg]

Awesome work, congrats on your first merged pull request!