Added 'array_contains' to the _COMPARISON_OPERATORS dict, to enable using array_contains in firestore queries.

[RohanTalip]

This appears to fix #6479.

[RohanTalip]

Prior to this change, I was getting this error:

...
  File "/project/appengine/main/venv3/lib/python3.6/site-packages/google/cloud/firestore_v1beta1/collection.py", line 215, in where
    return query.where(field_path, op_string, value)
  File "/project/appengine/main/venv3/lib/python3.6/site-packages/google/cloud/firestore_v1beta1/query.py", line 226, in where
    op=_enum_from_op_string(op_string),
  File "/project/appengine/main/venv3/lib/python3.6/site-packages/google/cloud/firestore_v1beta1/query.py", line 627, in _enum_from_op_string
    raise ValueError(msg)
ValueError: Operator string 'array_contains' is invalid. Valid choices are: <, <=, ==, >, >=.

With the change in this pull request patched locally, my query now appears to work.

[tseaver]

@RohanTalip Thanks for the patch! Can you please exercise the new operator in the unit test for '_enum_from_op_string'.

[RohanTalip]

@tseaver, done.

I don't see the circleci tests actually running any Python / pytest tests, but they passed locally:

$ cd firestore
$ virtualenv -p /usr/bin/python3 venv3
$ . venv3/bin/activate
$ pip install pytest
$ pip install mock
$ pip install -e .
$ pip install -e ../core/.
$ pytest
============================================================================== test session starts ===============================================================================
platform linux -- Python 3.6.5, pytest-3.10.1, py-1.7.0, pluggy-0.8.0
rootdir: /gitwork/googleapis/google-cloud-python/firestore, inifile:
collected 427 items                                                                                                                                                              

tests/unit/test__helpers.py .....................................................................................................................................          [ 31%]
tests/unit/test_batch.py ........                                                                                                                                          [ 33%]
tests/unit/test_client.py ...................................                                                                                                              [ 41%]
tests/unit/test_collection.py ..........................                                                                                                                   [ 47%]
tests/unit/test_cross_language.py .                                                                                                                                        [ 47%]
tests/unit/test_document.py ..............................................                                                                                                 [ 58%]
tests/unit/test_order.py ....                                                                                                                                              [ 59%]
tests/unit/test_query.py .......................................................                                                                                           [ 72%]
tests/unit/test_transaction.py ........................................                                                                                                    [ 81%]
tests/unit/test_watch.py .....................................................                                                                                             [ 93%]
tests/unit/gapic/v1beta1/test_firestore_client_v1beta1.py ..........................                                                                                       [100%]


================================================================================ warnings summary ================================================================================
... (lots of warnings)
==================================================================== 427 passed, 57 warnings in 9.94 seconds =====================================================================