feat(zarr): add thumbnail generation

[LDeakin]

Enabled by default. Uses the proposed Zarr thumbnail convention.

anu-ctlab-io ~/data/tomoLoRes.nc ~/data/tomoLoRes.zarr

.
├── 0
│   ├── c.0.0.0
│   └── zarr.json
├── thumbnails
│   ├── middle_xy_455x455.jpg
│   ├── middle_xz_455x204.jpg
│   └── middle_yz_455x204.jpg
└── zarr.json

zarr.json

  {
    "zarr_conventions": [
      {
        "schema_url": "https://raw.githubusercontent.com/zarr-conventions/thumbnails/refs/tags/v1/schema.json",
        "spec_url": "https://github.com/zarr-conventions/thumbnails/blob/v1/README.md",
        "uuid": "49326c01-1180-4743-b15f-f7157038a6ab",
        "name": "thumbnails",
        "description": "Metadata for thumbnails representing Zarr data"
      }
    ],
    "thumbnails": [
      {
        "width": 455,
        "height": 455,
        "media_type": "image/jpeg",
        "path": "thumbnails/middle_xy_455x455.jpg",
        "description": "Middle XY slice (downsampled)",
        "attributes": {
          "plane": "XY",
          "slice_axis": "z",
          "slice_index": 102,
          "plane_axes": [
            "y",
            "x"
          ],
          "lower_percentile": 1.0,
          "upper_percentile": 99.0,
          "lower_value": 10000.0,
          "upper_value": 15797.0,
          "full_resolution": false
        }
      },
      {
        "width": 455,
        "height": 204,
        "media_type": "image/jpeg",
        "path": "thumbnails/middle_xz_455x204.jpg",
        "description": "Middle XZ slice (downsampled)",
        "attributes": {
          "plane": "XZ",
          "slice_axis": "y",
          "slice_index": 227,
          "plane_axes": [
            "z",
            "x"
          ],
          "lower_percentile": 1.0,
          "upper_percentile": 99.0,
          "lower_value": 10000.0,
          "upper_value": 15797.0,
          "full_resolution": false
        }
      },
      {
        "width": 455,
        "height": 204,
        "media_type": "image/jpeg",
        "path": "thumbnails/middle_yz_455x204.jpg",
        "description": "Middle YZ slice (downsampled)",
        "attributes": {
          "plane": "YZ",
          "slice_axis": "x",
          "slice_index": 227,
          "plane_axes": [
            "z",
            "y"
          ],
          "lower_percentile": 1.0,
          "upper_percentile": 99.0,
          "lower_value": 10000.0,
          "upper_value": 15797.0,
          "full_resolution": false
        }
      }
    ]
  }