Allow event tracking to have action converted to srtring

What does this MR do and why?

Allows us to call tracking events and have our action converted to a string automatically since that is what the snowplow library expects.

• solves the current scenario where
  • we have an experiment and call track on it experiment(:some_experiment).track(:some_action) - we typically use symbols as this is allowed through this code
  • when we go to then clean up/promote the experiment to production, which removes the use of the experiment().track() dsl, we typically convert to a Gitlab::Tracking.event() call
  • it is easy to forget that we need to then convert action from a symbol to a string - this change solves that.

I'm not sure there is any reason not to do this as a convenience...

MR acceptance checklist

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