support protobuf

[lxning]

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Issue #, if available:

951

Description of changes:

1. define inference message proto

• add inference.proto
• update gradle to support protobuf build

2. support protobuf encode/decode in inference channel

• java code changes in worker data flow (ie. wlm dir)
• java code changes in http handler (ie. http dir)
• java code changes in tools (ie. util dir)

3. add unit test cases

• testPingProto
• testPredictionsProto
• testPredictionsModelNotFoundProto

4. fix mms-ci-build issues

• ci/Dockerfile.python3.6
• mms/tests/unit_tests/test_beckend_metric.py
• mms/tests/unit_tests/test_worker_service.py

Testing done:

Run unit test at local.

To run CI tests on your changes refer README.md

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.