Added beta support for Canvas SmartSearch API (Issue #659)

[alportoricensis]

Proposed Changes

• Added beta support for Canvas' SmartSearch API (https://canvas.instructure.com/doc/api/smart_search.html#SearchResult), as requested in Add API coverage for SmartSearch #659.
• Included a smartsearch method within course.py to handle requests to the SmartSearch API
• Created SearchResult subclass within searchresult.py to adapt different responses from the SmartSearch API (which can return a page, an assignment, etc.) to existing CanvasAPI classes (Page, Assignment, etc.). Took into account extensibility and raising ValueErrors as the API is experimental and breaking changes could occur.
• Added tests for both additions, maintaining 100% statement coverage

Fixes #659.

This was completed as part of Homework #6 of EECS 481, Software Engineering, at The University of Michigan, which requires students to create a pull request for an open-source project. For more information, see https://eecs481.org/hw6.html.