Band names for arrow exported images

[wiredfool]

Addresses #8329 (comment) . see also #8329 (comment)
Changes proposed in this pull request:

• Add the mode name as the schema name instead of "pixel"
• Add image band metadata in the arrow schema
• Tests for metadata interop
• Client testing with arro3 and nanoarrow similar to the pyarrow tests

For multiband images, we issue:

ArrowSchema:
  Format: FixedLengthArray[4]
  Children: 
    - ArrowSchema:
       Format: uint8
       name: RGB
       metadata: {image: {bands: [R, G, B, X]}}

This metadata is accessible using pa.array(img).type.field(0).metadata

For single band images we emit:

ArrowSchema:
  Format: ...
  name: F
  metadata: {image: {bands: [F]}}

This metadata is not apparently accessible via pyarrow or arro3, but is accessible via nanoarrow.

The ultimate goal is to be able to have:

• numpy/pandas/arrow users be able to do np.array(img) and be able to identify RGB channel names
• height/width would be ideal

@rok

[rok]

@paleolimbot am I right to say array metadata was not really designed to pass application metadata? What are your thoughts on what's being done here?
@wiredfool has the issue here that he can't access metadata of children when in certain cases.
I'm suggesting usage of FixedShapeTensorArray, but I imagine working with arrays directly would simplify things here.

[paleolimbot]

Cool!

It's true that field metadata at anything other the top-level struct has a medium chance of getting propagated through various Arrow operations. If you used the two metadata fields ARROW:extension:name (== pillow.image) and ARROW:extension:metadata as the JSON you're proposing above at the Array level, I think you will have more success getting this to propagate correctly. For pyarrow you'd have to implement a minimal ExtensionType class that a (pyarrow) user would have to "register" for that type to show up nicely in pyarrow.

Feel free to ping on anything if you have a question!