Structure peeks output to allow for adding additional data

[kaos]

          as we're getting more data into the `peek` output, I am more and more inclined to suggest we add more scoping on the data, to reduce the risk of name conflicts and also make it easier to trim/pick the desired data out of it.

Concretely what I'm suggesting is to have the actual target field values pushed down into fields for instance, so the peek output could look like:

[
  {
    "address": "src/python/pants/bin/remote_pants_runner.py",
    "target_type": "python_source",
    "fields": {
      "dependencies": [
        "src/python/pants/__init__.py",
        "src/python/pants/base/exiter.py",
        "src/python/pants/engine/internals/native_engine.pyi",
        "src/python/pants/option/global_options.py",
        "src/python/pants/option/options_bootstrapper.py",
        "src/python/pants/pantsd/pants_daemon_client.py"
      ],
      "description": null,
      "interpreter_constraints": null,
      "resolve": null,
      "restartable": false,
      "run_goal_use_sandbox": null,
      "skip_autoflake": false,
      "skip_black": false,
      "skip_docformatter": false,
      "skip_flake8": false,
      "skip_isort": false,
      "skip_mypy": false,
      "sources": [
        "src/python/pants/bin/remote_pants_runner.py"
      ],
      "tags": null
    },
    "raw_fields": {
      "dependencies": null,
      "source": "remote_pants_runner.py"
    },
    "sources_fingerprint": "...",
    "visibility": {
      "dependents_rules": ..,
      ..
    }
  },
  ..
]

maybe even write a schema for it's output format.. ;)

Originally posted by @kaos in #18383 (comment)

[kaos]

Offer the old output schema as an option to ease the transition.

pants peek --output-legacy-format