Introduce the Duo Chat callout for the global Duo Chat button

What does this MR do and why?

The MR introduces the DuoChatCallout component and its integration into the Duo Chat application.

Screenshots or screen recordings

Before
After

How to set up and validate locally

1. Set up GitLab Duo Chat
2. Load any page in the local GDK. There should neither global button, nor the callout, as represented in the screenshots, be presented on the page.
3. Enable the global GitLab Duo button:

# rails c

[1] pry(main)> Feature.enable(:tanuki_bot_breadcrumbs_entry_point)

3. Open any page and note the callout below the global GitLab Duo Chat button, as presented in the screenshots above

Just dismissing the callout

1. Dismiss the callout without opening chat by clicking ❌ in the top right corner of the callout. The callout gets dismissed. Chat should not be opened
2. Reload the page to make sure the callout is not shown while the button is still rendered

Clicking the "Ask GitLab Duo" button in the callout

1. Reset the dismiss of the callout:

# rails c
[2] pry(main)> Users::Callout.duo_chat_callout[0].destroy

2. Click the "Ask GitLab Duo" button in the callout. It should open the chat and dismiss the callout
3. Reload the page to make sure the callout is not shown while the button is still rendered

Clicking the global Duo Chat button

1. Reset the dismiss of the callout:

# rails c
[3] pry(main)> Users::Callout.duo_chat_callout[0].destroy

2. Reload the page to make sure the callout is shown again.
3. Click the global "GitLab Duo Chat" button. It should open the chat and dismiss the callout
4. Reload the page to make sure the callout is not shown while the button is still rendered

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #425954