Restore feature refs

[felixwang9817]

What this PR does / why we need it: #1691 allows for specifying FeatureServices in FeatureStore methods, but introduced a breaking change by renaming feature_refs to features. This PR ensures the change is not breaking, and inserts the appropriate DeprecationWarning.

Which issue(s) this PR fixes:

Fixes #

Does this PR introduce a user-facing change?:

Makes features an optional parameter in get_historical_features, instead of required

[codecov-commenter]

Codecov Report

Merging #1746 (3948acc) into master (2dfdace) will decrease coverage by 17.14%.
The diff coverage is 75.00%.

@@             Coverage Diff             @@
##           master    #1746       +/-   ##
===========================================
- Coverage   79.10%   61.95%   -17.15%     
===========================================
  Files          81       79        -2     
  Lines        6938     6855       -83     
===========================================
- Hits         5488     4247     -1241     
- Misses       1450     2608     +1158     

Flag	Coverage Δ
integrationtests	?
unittests	61.95% <75.00%> (+<0.01%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
...sts/integration/registration/test_feature_store.py	69.86% <ø> (-29.46%)	⬇️
sdk/python/feast/feature_store.py	91.00% <66.66%> (-2.64%)	⬇️
sdk/python/feast/feature_view.py	82.73% <100.00%> (-4.23%)	⬇️
.../integration/online_store/test_online_retrieval.py	17.39% <0.00%> (-82.61%)	⬇️
sdk/python/tests/utils/online_read_write_test.py	18.18% <0.00%> (-81.82%)	⬇️
...gration/registration/test_feature_service_apply.py	31.25% <0.00%> (-68.75%)	⬇️
...hon/tests/integration/registration/test_cli_gcp.py	31.70% <0.00%> (-68.30%)	⬇️
...n/tests/integration/registration/test_cli_redis.py	32.55% <0.00%> (-67.45%)	⬇️
sdk/python/feast/infra/online_stores/redis.py	30.35% <0.00%> (-64.29%)	⬇️
sdk/python/feast/infra/online_stores/datastore.py	31.03% <0.00%> (-62.07%)	⬇️
... and 32 more

---

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[felixwang9817]

/kind housekeeping

[tedhtchang]

/lgtm

[feast-ci-bot]

@tedhtchang: changing LGTM is restricted to collaborators

In response to this:

/lgtm

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[achals]

/lgtm

[feast-ci-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: achals, felixwang9817

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