Global Styles: allow users to store duotone data

[oandregal]

Follow-up to #34667
Related #35228 #35248 #35255

With this PR, duotone data provided by the users in the proper format will be stored safely in the CPT that holds the user data. This has no effect on the UI because we still don't have duotone in the global styles sidebar. When we add it, it'll work fine.

How to test

Test duotone works as expected:

• Use a theme that provides a theme.json and add within styles.blocks the following code:

"core/image": {
  "filter": {
    "duotone": "var(--wp--preset--duotone--blue-red)"
  }
}

• Add two images in the post editor and set a duotone filter for one. Leave the other in the default state.
• Publish the post and verify that both images have the proper filters applied in the post editor and in the front of the site.

Test that the user data filter works as expected:

• Run npm run test-php should pass.