Add distinct function to MongoHook in apache-airflow-providers-mongo

[octaviolage]

---

This PR adds a distinct() function to the MongoHook in apache-airflow-providers-mongo, which reffers pymongo.collection.distinct. This function allows users to return a list of distinct values from a MongoDB collection. I think the distinct function fits well with the other collection functions in the hook.

Before:

hook = MongoHook(conn_id='my_conn_id')
coll = hook.get_collection(
                mongo_db='test',
                mongo_collection='test',
            )
distinct_keys = coll.distinct('name')

After:

hook = MongoHook(conn_id='my_conn_id')
distinct_keys= hook.get_collection(
                mongo_db='test',
                mongo_collection='test',
                key='name'
            )

[boring-cyborg]

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[vincbeck]

There are some test failures. Please run static checks and also check the unit test you wrote, it is failing.

[vincbeck]

There are still some static checks failures. You should be able to fix them automatically by looking at documentation

[boring-cyborg]

Awesome work, congrats on your first merged pull request! You are invited to check our Issue Tracker for additional contributions.