Multi-channel segmentations

## Context
In the curation UI, we currently have support for selecting a channel to use from a multi-channel segmentation. However, on Cyto-DL's side, the curation channel is hard-coded to 0.

First off, we need to decide if we want to support multi-channel segmentations.

If we choose to support, we need to make adjustments in cyto-dl to expose overrides and adjustments in plugin to change those overrides. Additionally, we would need to make changes to the training UI to select segmentation channel as well as raw channel OR save the raw/segmentation channels selected during curation and automatically use those for training.

If we choose not to support, we would need to delete the channel selection dropdowns for segmentations in the curation UI. We would also need a strategy to handle the case in which the user tries to use a multi-channel segmentation (maybe just warn that channel 0 will be used by default?).