Implement basic publish/subscribe functionality

[nightduck]

• [x] rmw_create_publisher - This is where topic buffers are created/modified, and where allocator preference is specified
• [x] rmw_destroy_publisher - Cleanup from above
• [x] rmw_borrow_loaned_message - Make call to allocator's allocate function
• [x] rmw_return_loaned_message_from_publisher - Make call to allocator's deallocate function
• [x] rmw_publish_loaned_message - Modifies flags in topic buffer to indicate data is ready
• [x] rmw_create_subscription - Subscription created, topic buffers created/modified
• [x] rmw_destroy_subscription - Cleanup from above
• [x] rmw_subscription_count_matched_publishers - Return number of publishers on topic
• [x] rmw_publisher_count_matched_subscriptions - Return number of subscriptions on topic
• [x] rmw_take_loaned_message - The one we actually care above, returns pointer to topic buffer entry
• [x] rmw_take_loaned_message_with_info - Like above but more so
• [x] rmw_return_loaned_message_from_subscription - Takes pointer back, indicates message as read, possibly calling ublisher's deallocate function
• [x] rmw_publisher_get_actual_qos - self-explanatory
• [x] rmw_subscription_get_actual_qos - Self-explanatory
• [x] rmw_get_gid_for_publisher - Gets unique ID (GID) for publisher
• [x] rmw_compare_gids_equal - Compares unique IDs for publishers

Unit tests from test_rmw_implementation to pass

• [x] test_publisher
• [x] test_subscription
• [x] test_unique_identifiers

[nightduck]

Added dummy holders a67145fe7667d9e8e5f1c7f6a02d73475fdde15b

[nightduck]

Can create subscriptions and publishers e1be284d0bff82b036a8316cd475cd8d2daf5da2

[nightduck]

Publisher gids 3c23756f6159431579f32545fda708cd2f9e78ad

[nightduck]

Added cpp typesupport to serialization 6a070c14cee2539592926916b2d21c05bd54bb16