[ads] Custom notification ads

[tmancey]

Q2/2024:

• [ ] [ads] Icon and close button isn't aligned within the notification
• [ ] [ads] Educate users on the mobility and stickiness of custom push notifications
• [ ] [ads] Wrong custom notification ad theme when switching between private/normal windows
• [ ] [ads] Implement new custom notification ad design

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https://github.com/brave/brave-browser/projects/26#column-19651415