Pref structure for custom background

[sangwoo108]

sub task of #15252 .

This can block other sub tasks for custom background:

• Add an option to set solid color background for NTP #23255

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According to #15252, there're 4 options for users.

• custom files
• brave backgrounds
• solid colors
• gradient colors

and there's users can select a specific item from a collection or use random image from a collection.
We should decide pref structures for these.

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draft to discuss - done

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[
   kNTPBackground: {
      type: ‘brave’ | ‘custom_mage’ | ‘solid_color’ | ‘gradient_olor’
      random: bool  // indicates that we should pick one every time from |selected_type| collection
      selected_value?: string  // url or css value
  }
]