Make serverless alpha feature

[felixwang9817]

What this PR does / why we need it: This PR makes AWS Lambda serverless deployments an alpha feature. It also ensures that the AWS Lambda feature server can only be deployed if the registry is on S3.

Which issue(s) this PR fixes:

Fixes #

Does this PR introduce a user-facing change?:

Makes AWS Lambda feature server an alpha feature

[codecov-commenter]

Codecov Report

Merging #1928 (4a71caf) into master (a0918bf) will decrease coverage by 0.03%.
The diff coverage is 46.66%.

@@            Coverage Diff             @@
##           master    #1928      +/-   ##
==========================================
- Coverage   81.98%   81.95%   -0.04%     
==========================================
  Files          96       96              
  Lines        7542     7557      +15     
==========================================
+ Hits         6183     6193      +10     
- Misses       1359     1364       +5     

Flag	Coverage Δ
integrationtests	73.62% <46.66%> (-0.04%)	⬇️
unittests	59.93% <46.66%> (-0.03%)	⬇️

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

Impacted Files	Coverage Δ
sdk/python/feast/infra/aws.py	39.67% <33.33%> (-0.33%)	⬇️
sdk/python/feast/flags_helper.py	89.47% <50.00%> (-4.65%)	⬇️
sdk/python/feast/errors.py	70.00% <66.66%> (-0.08%)	⬇️
sdk/python/feast/flags.py	100.00% <100.00%> (ø)
sdk/python/feast/infra/online_stores/datastore.py	93.44% <0.00%> (+2.45%)	⬆️

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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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• OWNERS [achals,felixwang9817]

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[felixwang9817]

/kind feature