[code-reflection] RFR: Add tensor factories that accept an external arena

[Maurizio Cimadamore]

This PR adds three overloaded `Tensor::ofShape` factories which take an external arena. The existing factories have been rewired to call the new ones with a new automatic arena.

This allows us to make a small improvement in the MNISTDemo, where we can now allocate a tensor whose lifetime is managed by the arena that is also used to manage the lifetime of all the resources created within onnx session.

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Commit messages:
 - Initial push

Changes: https://git.openjdk.org/babylon/pull/335/files
  Webrev: https://webrevs.openjdk.org/?repo=babylon&pr=335&range=00
  Stats: 21 lines in 2 files changed: 15 ins; 2 del; 4 mod
  Patch: https://git.openjdk.org/babylon/pull/335.diff
  Fetch: git fetch https://git.openjdk.org/babylon.git pull/335/head:pull/335

PR: https://git.openjdk.org/babylon/pull/335