Support user-side integration of additional templates and annotations

[jcfr]

What's the problem this feature will solve?

Currently, adding support for new types of atlases, such as the macaque atlas (see Issue #229), requires engineering effort. Users are unable to independently load and visualize other atlas types without modifying the codebase, which restricts flexibility and usability for diverse research needs.

Describe the solution you'd like

Enable users to integrate and visualize new atlases, such as the macaque CCF, without requiring codebase modifications. This can be achieved by implementing the following steps:

1. Finalize Coordinate System Standardization: Complete the ongoing work to standardize the coordinate system, led by @lydiang. See Add support for updated Mouse CCF #230

2. Evaluate and Simplify Color Table Mapping: Determine if the "compacting" of color tables and mapping files (<atlas_type>_annotation_color_(slicer2allen|allen2slicer)_mapping.json) remains necessary after integrating ENH: Increase maximum number of colors loaded from colortable files Slicer/Slicer#6358.

3. Design a JSON schema to describe user-specified CCF templates and annotation files. This schema should include:

   • File paths for the template and annotations.
   • Coordinate system details.
   • Metadata for proper visualization.

4. Enhance Atlas Loading Logic: Update the Cell Locator loading mechanism to:

   • Parse the user-provided JSON configuration.
   • Display the corresponding atlas and annotations seamlessly.

Alternative Solutions
NA

Additional context

• Add support for macaque CCF  #229
• Add support for updated Mouse CCF #230

[allemangD]

Will need to add some atlas metadata to the root level of the JSON file. Key parts are atlas ID and checksum.

Need to isolate the "atlas loading" logic into some class, probably AtlasLogic or similar, that Annotations.fromDict can use to create its own atlas rather than depending on HomeWidget.logic. See #209 (comment).