feat: [FC-7879] add signal handler to save assignment dates to ContentDate model

[andrii-hantkovskyi]

NOTE: Depends on #324

Description

Adds a new signal handler to support saving assignment due dates to the ContentDate model in the edx-when service.

Key Changes:

• Introduces the _Assignment data class to standardize assignment metadata (title, date, block key, type).
• Implements update_or_create_assignments_due_dates() to:
  • Create or update ContentDate entries with assignment titles, dates, and metadata.
  • Store related information like block_type, subsection_name, and course_name.
  • Ensure DatePolicy entries are reused or created as needed.
• Adds logging and input validation to handle incomplete or invalid assignment data gracefully.

Testing:

Includes a full test suite verifying:

• New entries are inserted correctly.
• Existing entries are updated when data changes.
• Graceful handling of:
  • Assignments with missing title or date.
  • Multiple assignments at once.
  • Failures during DatePolicy creation.

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