Introduce an OnlineStore interface

[achals]

Signed-off-by: Achal Shah achals@gmail.com

What this PR does / why we need it:
Similar to how we decoupled the the OfflineStore for extensibility, this PR pulls out the OnlineStore. It also updates tests, and refactors the Sqlite implementation and GCP into their own components.

Which issue(s) this PR fixes:

Fixes #1573

Does this PR introduce a user-facing change?:

Introduce an OnlineStore interface.

[feast-ci-bot]

Skipping CI for Draft Pull Request.
If you want CI signal for your change, please convert it to an actual PR.
You can still manually trigger a test run with /test all

[codecov-commenter]

Codecov Report

Merging #1628 (6367767) into master (0d7e858) will decrease coverage by 0.01%.
The diff coverage is 86.79%.

@@            Coverage Diff             @@
##           master    #1628      +/-   ##
==========================================
- Coverage   83.59%   83.58%   -0.02%     
==========================================
  Files          67       71       +4     
  Lines        6017     6104      +87     
==========================================
+ Hits         5030     5102      +72     
- Misses        987     1002      +15     

Flag	Coverage Δ
integrationtests	83.50% <86.79%> (-0.02%)	⬇️
unittests	76.38% <82.55%> (-0.03%)	⬇️

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

Impacted Files	Coverage Δ
sdk/python/feast/feature_store.py	92.95% <ø> (ø)
sdk/python/tests/online_read_write_test.py	100.00% <ø> (ø)
sdk/python/tests/test_cli_redis.py	100.00% <ø> (ø)
...hon/tests/test_offline_online_store_consistency.py	100.00% <ø> (ø)
sdk/python/tests/test_online_retrieval.py	100.00% <ø> (ø)
sdk/python/feast/errors.py	65.85% <66.66%> (+0.06%)	⬆️
sdk/python/feast/infra/online_stores/datastore.py	79.09% <79.09%> (ø)
sdk/python/feast/infra/online_stores/helpers.py	79.41% <79.41%> (ø)
...k/python/feast/infra/online_stores/online_store.py	80.95% <80.95%> (ø)
sdk/python/feast/infra/local.py	90.00% <88.88%> (-6.74%)	⬇️
... and 11 more

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[feast-ci-bot]

[APPROVALNOTIFIER] This PR is APPROVED

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

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[woop]

/lgtm