[code-reflection] RFR: OnnxRuntime and OnnxProtoBuilder continuation

[Adam Sotona]

OnnxRuntime and OnnxProtoBuilder continuation

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Commit messages:
 - improved SimpleTest to cover constants encoding in protobuf models
 - modeling of attributes (for constant op)
 - fixed protobuf encoding of longs
 - adding attributes to model building - work in progress
 - OnnxProtoBuilder comments + SimpleTest consolidation
 - added OnnxElementType::id, simplified OnnxProtoBuilder::buildFuncModel and consolidated RuntimeTest and SimnpleTest

Changes: https://git.openjdk.org/babylon/pull/316/files
  Webrev: https://webrevs.openjdk.org/?repo=babylon&pr=316&range=00
  Stats: 358 lines in 7 files changed: 223 ins; 91 del; 44 mod
  Patch: https://git.openjdk.org/babylon/pull/316.diff
  Fetch: git fetch https://git.openjdk.org/babylon.git pull/316/head:pull/316

PR: https://git.openjdk.org/babylon/pull/316