feat(GlDuoChatLoader): added component

What does this MR do?

The adds a new GlDuoChatLoader component. The component represents the loading process in Duo Chat, showing the active tool, working on finding an answer.

The component itself is not new and is currently used in GitLab Duo Chat at gitlab.com. The task of this MR is to move the loader into the gitlab-ui component. This is one of the steps in the course of Shared UI for GitLab Duo chat (&11263) activity, and in particular Add the GitLab Duo chat component to @gitlab/ui (gitlab#422706 - closed)

To get the rationale behind why this needs to be done, please consult Add the GitLab Duo chat component to @gitlab/ui (gitlab#422706 - closed)

Note: this is not a public component and will only be used in the context of the complete GlDuoChat component which will be public. Hence this component is not added to the exports in src/index.js.

Screenshots or screen recordings

Integration merge requests

• GitLab: mr_url
• CustomersDot: mr_url
• Status Page: mr_url

Does this MR meet the acceptance criteria?

This checklist encourages the authors, reviewers, and maintainers of merge requests (MRs) to confirm changes were analyzed for conformity with the project's guidelines, security and accessibility.

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Conformity

• Code review guidelines.
• GitLab UI's contributing guidelines.
• If it changes a Pajamas-compliant component's look & feel, the MR has been reviewed by a UX designer.
• If it changes GitLab UI's documentation guidelines, the MR has been reviewed by a Technical Writer.
• If the MR changes a component's API, integration MR(s) have been opened (see integration merge requests above).
• Added the ~"component:*" label(s) if applicable.

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• Security reports checked/validated by a reviewer from the AppSec team

Accessibility

If this MR adds or modifies a component, take a few moments to review the following:

• All actions and functionality can be done with a keyboard.
• Links, buttons, and controls have a visible focus state.
• All content is presented in text or with a text equivalent. For example, alt text for SVG, or aria-label for icons that have meaning or perform actions.
• Changes in a component’s state are announced by a screen reader. For example, changing aria-expanded="false" to aria-expanded="true" when an accordion is expanded.
• Color combinations have sufficient contrast.