Improving ergonomics for channel names handling

[LucaMarconato]

Brainstormed together with @minhtien-trinh.

Working with channel names in multi-channel images in not straightforward. In particular:

• [x] the c_coords argument in Image2DModel.parse() is too hidden; we should move it out of kwargs
• [x] we could consider adding an helper function set_channels() (to complement get_channels())
• [x] SpatialData.write_metadata() currently doesn't handle channel names, so it is not possible to easily overwrite the channel names metadata without rewriting the heavy raster data.

[melonora]

just waiting for review @minhtien-trinh

[minhtien-trinh]

Looks great to me, thank you for handling this @melonora