Add instance banners support (#6372)

Fixes
https://linear.app/sourcegraph/issue/CODY-4104/[morgan-stanley]-ability-to-show-a-message-with-a-disclaimer-on-the-ui

This PR adds support for instance level banners in cody chat UI. It
renders up to 240 symbols and supports markdown syntax. This is the
first iteration of this just to fulfil CODY-4104 needs.

Message and title are customizable by instance global settings, via
`cody.notices` fields.

For instance, configuration for message from the screenshots 

```
 "cody.notices": [
    {
      "key": "testing-banner-key-2",
      "title": "Hi Sourcegraph!",
      "message": "This is testing message to test our new custom message banner, say hi to Felix from me. [Happy holidays](https://www.youtube.com/watch?v=E8gmARGvPlI) \n And special shout out to Search team."
    }
  ],
```

| Cody Web | VSCode |
| --------- | --------- | 
| <img width="1223" alt="Screenshot 2024-12-16 at 20 08 12"
src="https://github.com/user-attachments/assets/2cac18b3-9b31-4662-a5a9-866b8461181c"
/> | <img width="1092" alt="Screenshot 2024-12-16 at 20 06 00"
src="https://github.com/user-attachments/assets/221b7f3c-d089-4041-be5e-11b69b4e6f3f"
/> |
 
## Test plan
- Run local vs code extension connected to s2; you should see the banner
as shown in the screenshots above.