resolve environment variables in repo config

[samuel100]

What this PR does / why we need it:
The feature_store.yaml file allows Feast users to enter connection strings for offline and online stores. For example:

registry:
  path: ./registry.db
project: production
provider: local
online_store:
  type: redis
  connection_string: my-secret-connection-string

This means that a connection string can sit in a plain text file, which makes CI/CD impossible.

This PR allows Feast users to hide the connection string in the YAML file by using environment variables. For example:

registry:
  path: ./registry.db
project: production
provider: local
online_store:
  type: redis
  connection_string: ${CONN_STRING_ENV_VAR}

Here the environment variable CONN_STRING_ENV_VAR is resolved in repo_config.load_repo_config method. This aids CI/CD flows (e.g. using secrets in GitHub actions).

Does this PR introduce a user-facing change?:

NONE

[feast-ci-bot]

Hi @samuel100. Thanks for your PR.

I'm waiting for a feast-dev member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work. Regular contributors should join the org to skip this step.

Once the patch is verified, the new status will be reflected by the ok-to-test label.

I understand the commands that are listed here.

Instructions for interacting with me using PR comments are available here. If you have questions or suggestions related to my behavior, please file an issue against the kubernetes/test-infra repository.

[achals]

/ok-to-test

[codecov-commenter]

Codecov Report

Merging #1909 (0433459) into master (dcae160) will increase coverage by 22.44%.
The diff coverage is 100.00%.

@@             Coverage Diff             @@
##           master    #1909       +/-   ##
===========================================
+ Coverage   60.60%   83.05%   +22.44%     
===========================================
  Files          96       96               
  Lines        7316     7317        +1     
===========================================
+ Hits         4434     6077     +1643     
+ Misses       2882     1240     -1642     

Flag	Coverage Δ
integrationtests	74.80% <100.00%> (?)
unittests	60.61% <100.00%> (+<0.01%)	⬆️

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

Impacted Files	Coverage Δ
sdk/python/feast/repo_config.py	78.97% <100.00%> (+4.12%)	⬆️
sdk/python/feast/online_response.py	86.79% <0.00%> (+1.88%)	⬆️
sdk/python/feast/infra/online_stores/sqlite.py	96.51% <0.00%> (+2.32%)	⬆️
sdk/python/feast/usage.py	66.66% <0.00%> (+2.77%)	⬆️
sdk/python/feast/feature.py	72.72% <0.00%> (+3.03%)	⬆️
sdk/python/feast/feature_view.py	86.57% <0.00%> (+5.36%)	⬆️
sdk/python/feast/errors.py	70.31% <0.00%> (+5.46%)	⬆️
sdk/python/feast/flags_helper.py	94.11% <0.00%> (+5.88%)	⬆️
sdk/python/feast/infra/provider.py	90.67% <0.00%> (+5.93%)	⬆️
... and 46 more

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[adchia]

/lgtm

[feast-ci-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: adchia, samuel100

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