Fix FormulaEngine typing (#1011)

With this PR, all the typing ambiguities we had in `formula_engine.py`
have been fixed.

Author: shsms

RELEASE_NOTES.md

@@ -39,3 +39,4 @@
 - Fixed a bug in the code examples in the getting-started tutorial.
 - Fixed a bug in `ConfigManagingActor` that was not properly comparing the event path to the config file path when the config file is a relative path.
 - Re-expose `ComponentMetricId` to the docs.
+- Fixed typing ambiguities when building composite formulas on streaming data.