[AIChat] - Chat Contextual Menu

[Brandon-T]

Feature Overview

• The chat as a contextual menu that allows the user to switch models on the fly, go premium, suggest questions, and access settings

Implementation Details

• Implement the popover menu
• Add the models to the popover menu and allow the user to select models on the fly
• Add the ability to visit settings
• Add the ability to go premium from the menu

[kjozwiak]

As per brave/brave-browser#36585 (comment) & https://bravesoftware.slack.com/archives/C04KPE0BUDU/p1709872123838319?thread_ts=1709830872.231289&cid=C04KPE0BUDU, going to close this one off as QA/No and release-notes/exclude as we'll use brave/brave-browser#36585 as a guide/main issue to run through Leo via 1.63.x.