[code-reflection] RFR: Onnx model and protobuf model improvements

[Adam Sotona]

Onnx model and protobuf model improvements.
Onnx protobuf constants, builder and model generated.

-------------

Commit messages:
 - ProtoGen work in progress
 - ProtoGen work in progress
 - ProtoGen work in progress
 - ProtoGen work in progress
 - ProtoGen work in progress
 - ProtoGen work in progress
 - implementation of ProtoGen
 - OnnxProtoBuilder separation of the generated code
 - Towards a fully generated OnnxProtoBuilder
 - AttributeType.TENSOR holds Tensor instead of byte array, AttributeType.TENSOR holds array of Tensors

Changes: https://git.openjdk.org/babylon/pull/414/files
  Webrev: https://webrevs.openjdk.org/?repo=babylon&pr=414&range=00
  Stats: 4342 lines in 13 files changed: 3192 ins; 1042 del; 108 mod
  Patch: https://git.openjdk.org/babylon/pull/414.diff
  Fetch: git fetch https://git.openjdk.org/babylon.git pull/414/head:pull/414

PR: https://git.openjdk.org/babylon/pull/414