Add custom data sources

[achals]

What this PR does / why we need it:

After allowing for custom online stores and offline stores, we need to update the DataSource API so that implementors can create custom. DataSources to be used in their feature repo, and provide a mechanism through which these DataSources can be serialized/deserialized from the registry.

This is done by introducing a super simple CustomSourceOptions message that contains information about the class used for the source, and any serialized contents. Implementors are responsible for defining a to_proto and from_proto method and they can serialize the contents into this class as they need.

Which issue(s) this PR fixes:

Fixes #

Does this PR introduce a user-facing change?:

Allow for custom Data Sources to be defined.

[codecov-commenter]

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Codecov Report

Attention: Patch coverage is 79.47883% with 63 lines in your changes missing coverage. Please review.

Project coverage is 84.47%. Comparing base (c5300cb) to head (9e3f4e3).
Report is 1737 commits behind head on master.

Files with missing lines	Patch %	Lines
sdk/python/feast/infra/offline_stores/redshift.py	67.44%	28 Missing ⚠️
sdk/python/feast/infra/offline_stores/bigquery.py	80.23%	17 Missing ⚠️
sdk/python/feast/data_source.py	74.19%	8 Missing ⚠️
sdk/python/feast/infra/offline_stores/file.py	88.57%	8 Missing ⚠️
sdk/python/feast/feature_view.py	66.66%	2 Missing ⚠️

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Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1713      +/-   ##
==========================================
+ Coverage   84.45%   84.47%   +0.01%     
==========================================
  Files          79       79              
  Lines        7012     7071      +59     
==========================================
+ Hits         5922     5973      +51     
- Misses       1090     1098       +8     

Flag	Coverage Δ
integrationtests	84.40% <79.47%> (+0.01%)	⬆️
unittests	69.45% <62.54%> (+0.13%)	⬆️

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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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• OWNERS [achals,woop]

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[woop]

Just waiting for merge conflict to be resolved. Otherwise lgtm.