fix: publish diagnostic_agg using a timer, not wall_timer

[MrBlenny]

Similar to #458

diagnostics_agg should be published using a timer rather than wall_timer so it respects use_sim_time

[redvinaa]

This doesn't solve the problem, because in the aggregator all status items create their own default clock.

diagnostics/diagnostic_aggregator/src/status_item.cpp

Line 49 in 61211e6

: clock_(new rclcpp::Clock())

[ct2034]

@MrBlenny can you please open an issue, describe the problem you want to fix and how to reproduce it.

[ct2034]

see comments above