[Feature] Add list_user_activity_stream() to vvcanvas

[tin900]

Summary:
Implement a function to fetch the recent activity feed for a user.

What to implement:

• Function: list_user_activity_stream(user_id = "self", ...)
• Endpoint: GET /api/v1/users/:user_id/activity_stream
• Optional params:
  • Pagination parameters (as per package conventions)
• Should default to "self" for current authorized user when user_id is not provided.
• Returns: List or dataframe of activity stream objects (Canvas Stories).

Design notes:

• Consistent output format and param handling as other user-facing vvcanvas methods.
• Usage example in Roxygen docs. Reference Canvas API endpoint.
• Add as new item in user-centric section of pkgdown structure.

To update:

• Add to TODO.md
• Register in _pkgdown.yml, ideally under a "Users" or "Activity" section.

[linear]

EDU-164 [Feature] Add list_user_activity_stream() to vvcanvas