Google sheets plugin - image alt text support

[djohalo2]

Description

Adds image alt-text support to the Google Sheets plugin, including field mapping, synchronization, persisted assignments, and automatic cleanup when image field types change.

google-sheets-plugin-alt-text.mov

Fixes: #423

Changelog

• Added support for mapping Google Sheets columns as alt text for CMS image fields.

Testing

• Image alt text imports correctly
  • Map a text column to an image field as Alt Text
  • Confirm no CMS field is created for the alt-text column
  • Confirm the rendered image has the expected alt value
• Alt-text mappings persist and update
  • Reopen the plugin and confirm the mapping is restored
  • Change the alt text and run automatic sync
  • Confirm the rendered alt text updates
• Alt text can be cleared
  • Sync an image with populated alt text
  • Clear the corresponding sheet cell and sync again
  • Confirm the previous alt text is removed
• Existing collections remain compatible after upgrading
  • Sync without changing field mappings
  • Confirm existing fields, items, images, and ignored columns remain unchanged
• Mapping changes are handled safely
  • Remap the alt-text column to another image
  • Change the target image to another field type
  • Confirm no stale or incorrect mapping remains
• Sheet structure changes are handled safely
  • Reorder the mapped columns
  • Rename or remove a mapped column
  • Confirm existing CMS fields are not corrupted