Add ButtonIcon as Toast component option for PNA25 Toast message

### What is this about?

The PNA25 (Privacy Notice Announcement) toast currently uses a Primary Button with an 'x' instead of an Icon Button as the close button as this was not available in the Toast component. The Toast component should be updated to support Icon Buttons as close buttons so that the PNA25 toast can match the expected design.

### Scenario

- GIVEN the `extension-ux-pna25` remote feature flag is on
- WHEN an existing user logs in to MM
- THEN they should see a toast message with privacy update messaging and a Close Icon Button in the top right corner

### Design

_No response_

### Technical Details

_No response_

### Threat Modeling Framework

_No response_

### Acceptance Criteria

_No response_

### Stakeholder review needed before the work gets merged

- [ ] Engineering (needed in most cases)
- [ ] Design
- [ ] Product
- [ ] QA (automation tests are required to pass before merging PRs but not all changes are covered by automation tests - please review if QA is needed beyond automation tests)
- [ ] Security
- [ ] Legal
- [ ] Marketing
- [ ] Management (please specify)
- [ ] Other (please specify)

### References

Start by locating the Toast component and the PNA25 implementation that is enabled by the extension-ux-pna25 remote feature flag. Trace how the current Primary Button close control is configured, then verify the existing-user login scenario on mobile and confirm that the toast displays a Close Icon Button in the top right.

Written by the indexing model from the issue text.