feat: Added Spark support for Delta and Avro

[creativedutchmen]

When using spark as your offline store, it's useful to be able to read from Delta tables, so that you get a consistent view of the data, even when writing and reading at the same time (eg when using Spark Streaming to stream updates to the table, and materialising every 10 minutes).

Since we're calling spark_session.read.format(<FORMAT>), which is the preferred way of reading delta and Avro, this should work without further changes.

I've added Avro support because some (cloud) products support writing the contents of a topic to Avro automatically - being able to materialise from this persisted stream could be useful if you don't feel like building it yourself.

[codecov-commenter]

Codecov Report

Merging #2757 (89579b7) into master (0d195c4) will decrease coverage by 0.04%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #2757      +/-   ##
==========================================
- Coverage   80.59%   80.55%   -0.05%     
==========================================
  Files         173      173              
  Lines       15006    15008       +2     
==========================================
- Hits        12094    12089       -5     
- Misses       2912     2919       +7     

Flag	Coverage Δ
integrationtests	70.65% <100.00%> (-0.38%)	⬇️
unittests	59.52% <100.00%> (+<0.01%)	⬆️

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

Impacted Files	Coverage Δ
...stores/contrib/spark_offline_store/spark_source.py	61.15% <100.00%> (+0.56%)	⬆️
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%)	⬇️
...ython/feast/embedded_go/online_features_service.py	89.14% <0.00%> (-0.78%)	⬇️

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[adchia]

/lgtm

[feast-ci-bot]

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