Fix scorer gradient direction semantics

[V-SK]

Summary

• align scorer sparse submission scoring with PS gradient descent semantics by applying learning rate
• require and validate learning_rate in score requests and cover the behavior with a regression test
• avoid blocking scorer HTTP startup on synchronous PS endpoint registration

Testing

• python3 -m unittest -v scorer/test_score_direction.py
• python3 -m py_compile scorer/scoring_server.py scorer/test_score_direction.py