feat: add new API to retrieve all components in a unit

[viv-helix]

Description

• This PR adds a new API endpoint that retrieves all components within a unit, returning unit-level metadata along with a list of contained components, including their block IDs, block types, and display names.

• Access to this endpoint is gated by a new course-level waffle flag contentstore.enable_unit_expanded_view that controls the Unit Expanded View feature in Studio’s Course Outline. When the flag is disabled for a course, requests to retrieve unit components are blocked with a 403 Forbidden response. When the flag is enabled, the endpoint functions normally.

URL: GET /api/contentstore/v1/unit_handler/{usage_key_string}

Jira

• https://2u-internal.atlassian.net/browse/TNL2-473

Testing

• Verified successful response for valid unit usage keys
• Confirmed correct component metadata is returned
• Ensured proper HTTP status codes for success and error cases