Make context.on_shutdown allow free functions as shutdown callbacks

[sloretz]

Split from #858

WeakMethod throws an exception if the callable is not a bound method, which prevented using free functions as a shutdown callback. This allows free functions by storing them with weakref.ref.

Example exception before this PR

9: >           raise TypeError("argument should be a bound method, not {}"
9:                             .format(type(meth))) from None
9: E           TypeError: argument should be a bound method, not <class 'function'>

[j-rivero]

looks great.

[sloretz]

CI (build: --packages-above-and-dependencies rclpy test: --packages-above rclpy)

• Linux
• Linux-aarch64
• macOS
• Windows

[fujitatomoya]

lgtm