Track and Persist source types for triggers

What does this MR do and why?

Track and Persist source type for triggers.

New source_type values: flow_trigger

Specific values for flow triggers will not be persisted in source_type, but will rather be in trigger_event_type, added in !255769 (merged)

References

#629614 (closed)

How to set up and validate locally

1. Run a developer flow in your GDK, e.g. in an MR say @duo-developer-gitlab-duo - Who invented sarcasm?
2. In the rails console, confirm the trigger was saved:

pry(main)> w = Ai::DuoWorkflows::Workflow.last.source_type

=> "flow_trigger"

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