Add fire_immediately flag parameter for timers

- Time event will be fired at the `start` instant, then every interval thereafter
- Closes #2600

Author: cjdsellers

RELEASES.md

@@ -3,6 +3,7 @@
 Released on TBD (UTC).
 
 ### Enhancements
+- Added `fire_immediately` flag parameter for timers where a time event will be fired at the `start` instant and then every interval thereafter (default `False` to retain current behavior) (#2600), thanks for the idea @stastnypremysl
 - Added `time_bars_build_delay` config option for `DataEngineConfig` (#2676), thanks @faysou
 - Added support for DEX swaps for blockchain adapter (#2683), thanks @filipmacek