feat: [COSMO2-821] Add plugin slot for access expiration top banner

[santhosh-apphelix-2u]

Summary

Adds a new PluginSlot in the Learning MFE for the learner-facing "Audit Access Expires" top banner.

What changed

• Added a new top banner PluginSlot wrapper
• Mounted the slot in the Learning MFE page shell where the banner should appear
• Updated plugin slot documentation

Why

The existing AccessExpirationAlert component is present in the MFE but is not wired into the UI.
This change introduces a dedicated PluginSlot so the banner can be rendered through the plugin architecture.

Notes

• This PR does not render the banner by itself
• Actual banner UI is provided by the ads plugin
• Host config wiring is handled separately in edx-internal

[Copilot]

Pull request overview

Adds a new plugin slot to the courseware page shell so plugin authors can render a banner in the Learning MFE chrome above the sequence content.

Changes:

• Introduces CoursewareTopBannerSlot (org.openedx.frontend.learning.courseware_top_banner.v1) with a documented alias and expected plugin props.
• Renders the new slot in src/courseware/course/Course.jsx just above sequence alerts/content.
• Updates the plugin-slots index README to include the new slot.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated no comments.

File	Description
src/plugin-slots/README.md	Adds the new slot to the centralized plugin slot list for discoverability.
src/plugin-slots/CoursewareTopBannerSlot/README.md	Documents slot id, alias, props, and intended placement/use case.
src/plugin-slots/CoursewareTopBannerSlot/index.tsx	Implements the new PluginSlot wrapper component and exports it.
src/courseware/course/Course.jsx	Integrates the slot into the courseware page shell render tree.

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