Remove ament_cmake_ros dependency

[nvcyc]

The dependent ament_cmake_ros package transitively pulls in RMW-layer packages which is unnecessarily heavy to class_loader that is supposed to be an independent plugin loading library.

This PR removes the ament_cmake_ros dependency and replaces with a plain ament_cmake with an explicit SHARED library type to keep the dependency minimal.

As a side note, this fix is also required when introducing the rosidl::Buffer feature to the RMW layer where pluginlib will be used to load buffer backends.

Without this fix, the following circular dependency will be formed:
class_loader -> ament_cmake_ros -> rmw_test_fixture_implementation -> ... -> rmw_fastrtps_cpp -> rosidl_buffer_backend_registry -> pluginlib -> class_loader

[ahcorde]

Pulls: #226
Gist: https://gist.githubusercontent.com/ahcorde/a596624759d7541114bbc3454655df62/raw/6d32899428d89727662a8c4345bb2d0f780b6067/ros2.repos
BUILD args: --packages-above-and-dependencies class_loader
TEST args: --packages-above class_loader
ROS Distro: rolling
Job: ci_launcher
ci_launcher ran: https://ci.ros2.org/job/ci_launcher/18543

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