Docstring tests

[felixwang9817]

What this PR does / why we need it: Most of our important user-facing methods (apply, get_historical_features, etc.) have docstrings with interactive examples. This PR introduces tests for these interactive examples.

Which issue(s) this PR fixes:

Fixes #

Does this PR introduce a user-facing change?:

NONE

[codecov-commenter]

Codecov Report

Merging #1749 (4428bd6) into master (1841bb3) will increase coverage by 0.34%.
The diff coverage is 95.65%.

@@            Coverage Diff             @@
##           master    #1749      +/-   ##
==========================================
+ Coverage   83.95%   84.30%   +0.34%     
==========================================
  Files          73       76       +3     
  Lines        6003     6084      +81     
==========================================
+ Hits         5040     5129      +89     
+ Misses        963      955       -8     

Flag	Coverage Δ
integrationtests	84.22% <95.65%> (+0.34%)	⬆️
unittests	64.71% <95.65%> (+0.54%)	⬆️

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

Impacted Files	Coverage Δ
...k/python/feast/infra/offline_stores/file_source.py	85.91% <ø> (ø)
sdk/python/feast/infra/utils/aws_utils.py	91.26% <ø> (ø)
sdk/python/tests/doctest/test_all.py	95.12% <95.12%> (ø)
sdk/python/feast/feature_store.py	93.58% <100.00%> (+0.50%)	⬆️
sdk/python/feast/registry.py	77.94% <0.00%> (-1.41%)	⬇️
sdk/python/feast/feature_view.py	87.05% <0.00%> (ø)
sdk/python/feast/wait.py	23.52% <0.00%> (ø)
sdk/python/feast/constants.py	100.00% <0.00%> (ø)
... and 1 more

---

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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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