Add functions for resolving 'best available' QoS policies

[jacobperron]

Connects to https://github.com/ros2/rmw/pull/320

Given a QoS profile and set of endpoints for the same topic, overwrite any policies set to BEST_AVAILABLE with a policy such that it matches all endpoints while maintaining a high level of service.

Add testable functions for updating BEST_AVAILABLE policies,

• qos_profile_get_best_available_for_subscription
• qos_profile_get_best_available_for_publisher

and add convenience functions that actual query the graph for RMW implementations to use,

• qos_profile_get_best_available_for_topic_subscription
• qos_profile_get_best_available_for_topic_publisher

[jacobperron]

PTAL at the latest addition to handle services: ed492d9