feat: auto-detect custom start and stop events in workflow classes

[masci]

Description

With #17767 we introduced the ability to use custom start and stop events in workflow. I wasn't happy with the UX, because in order to use custom events you had to pass them to the workflow init like this (or a variation of this):

wf = MyWorkflow(start_event_class=MyStart, stop_event_class=MyStop)

With this PR, start and stop events are inferred from the step definitions, so if your workflow is using MyStart and MyStop, you can intantiate the workflow without caring about that: wf = MyWorkflow().

Question: the current PR is backward compatible, but start_event_class and stop_event_class were added very recently and not documented. Should we just kill them and always rely on introspection?

Type of Change

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• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

How Has This Been Tested?

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• I added new unit tests to cover this change
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