Refactor ColumnIndex APIs in Column Virtualization

[AmanGupta2708]

Description

1. Made column Index global to table instead of cell group, and modify context api accordingly.
2. Simplify column virtualisation api, o that it require only one getter function instead of 3 separate getter function for each cell group type.
3. Rename some file and variable name to make them more specific and less confusing, see https://github.com/schrodinger/fixed-data-table-2/commit/54fb5ba5101c5fa18f757a19e5ac450edb6c6754. (This branch is cut from column-virtualization-renaming so it won't show those changes)

ToDo -:

Test and fix plugins to work after this change.

Types of changes

• [ ] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [x] Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• [x] My code follows the code style of this project.
• [ ] My change requires a change to the documentation.
• [ ] I have updated the documentation accordingly.
• [ ] I have read the CONTRIBUTING document.