feat: [FC-7879] add REST API for user dates retrieval

[andrii-hantkovskyi]

NOTES: Depends on #326

Description

This PR introduces a new authenticated endpoint to retrieve all date values (default and user-specific overrides) for a given course and user.

Endpoints

GET /api/edx_when/v1/user-dates/
GET /api/edx_when/v1/user-dates/<course_id>

Query Parameters (optional)

• block_types: comma-separated list of block types to filter (e.g., sequential,vertical)
• block_keys: comma-separated list of specific block keys
• date_types: comma-separated list of date field types (e.g., due,start,end)

Response

• A dictionary mapping block keys to date values (ISO format).

{
  "block-v1:TestX+Test+2023+type@sequential+block@assignment_1": "2023-12-15T23:59:59Z",
  "block-v1:TestX+Test+2023+type@sequential+block@quiz_1": "2023-12-20T23:59:59Z"
}

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