Add feature server docs & small changes in local server

[tsotnet]

Signed-off-by: Tsotne Tabidze [email protected]

What this PR does / why we need it: This adds the feature server documentation to Feast. We also make a couple of small changes to the local server. Namely:

1. Change GET to POST request
2. Change routing from /get-online-features/ to /get-online-features, which doesn't change anything for the local server, but fixes a very ambiguous bug when using this server with AWS Lambda (which is WIP).
3. Add experimental warning on feast serve and add it in the CLI message as well.

Which issue(s) this PR fixes:

Fixes #

Does this PR introduce a user-facing change?:

Document local feature server (experimental)

[codecov-commenter]

Codecov Report

Merging #1852 (dd06ffc) into master (0dc13f0) will decrease coverage by 21.27%.
The diff coverage is 33.33%.

@@             Coverage Diff             @@
##           master    #1852       +/-   ##
===========================================
- Coverage   84.45%   63.17%   -21.28%     
===========================================
  Files          90       91        +1     
  Lines        6773     6963      +190     
===========================================
- Hits         5720     4399     -1321     
- Misses       1053     2564     +1511     

Flag	Coverage Δ
integrationtests	?
unittests	63.17% <33.33%> (-0.57%)	⬇️

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

Impacted Files	Coverage Δ
sdk/python/feast/cli.py	42.58% <ø> (ø)
sdk/python/feast/feature_server.py	27.77% <33.33%> (+1.30%)	⬆️
.../integration/online_store/test_universal_online.py	16.66% <0.00%> (-83.34%)	⬇️
.../integration/online_store/test_online_retrieval.py	17.39% <0.00%> (-82.61%)	⬇️
sdk/python/tests/utils/online_read_write_test.py	18.18% <0.00%> (-81.82%)	⬇️
...fline_store/test_universal_historical_retrieval.py	19.29% <0.00%> (-80.71%)	⬇️
...gration/registration/test_feature_service_apply.py	31.25% <0.00%> (-68.75%)	⬇️
sdk/python/feast/infra/online_stores/datastore.py	30.32% <0.00%> (-63.12%)	⬇️
sdk/python/feast/infra/online_stores/redis.py	30.35% <0.00%> (-62.50%)	⬇️
sdk/python/tests/data/data_creator.py	38.09% <0.00%> (-61.91%)	⬇️
... and 46 more

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[achals]

/lgtm

[feast-ci-bot]

[APPROVALNOTIFIER] This PR is APPROVED

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