fix: Change online UI schema field to be optional

[dvanbrug]

What this PR does / why we need it:
Change the online field in feastFeatureViews schema to be optional.
This addresses a problem with the UI erroring out when trying to present
offline FeatureViews since protobuf would not pass along the online
tag.

Which issue(s) this PR fixes:

Fixes #2719

[achals]

/ok-to-test
/lgtm

[codecov-commenter]

Codecov Report

Merging #2766 (d9b5e94) into master (da763c6) will decrease coverage by 0.06%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #2766      +/-   ##
==========================================
- Coverage   80.56%   80.50%   -0.07%     
==========================================
  Files         173      173              
  Lines       15093    15093              
==========================================
- Hits        12160    12150      -10     
- Misses       2933     2943      +10     

Flag	Coverage Δ
integrationtests	70.45% <ø> (-0.36%)	⬇️
unittests	59.60% <ø> (-0.02%)	⬇️

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

Impacted Files	Coverage Δ
sdk/python/tests/unit/test_feature_validation.py	66.66% <0.00%> (-16.67%)	⬇️
sdk/python/tests/utils/online_read_write_test.py	93.54% <0.00%> (-6.46%)	⬇️
...ion/registration/test_stream_feature_view_apply.py	92.85% <0.00%> (-3.58%)	⬇️
.../integration/online_store/test_online_retrieval.py	96.84% <0.00%> (-3.16%)	⬇️
...k/python/tests/unit/test_on_demand_feature_view.py	67.18% <0.00%> (-1.57%)	⬇️
...ython/feast/embedded_go/online_features_service.py	89.14% <0.00%> (-0.78%)	⬇️

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[adchia]

/lgtm

[feast-ci-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: achals, adchia, dvanbrug

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• OWNERS [achals,adchia]

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