Add DAST config variable usage metric definition

What does this MR do and why?

Define a DAST config variable usage metric as an internal event. This event will capture the configuration variables in use during DAST scans to understand usage, adoption, and when variables can be safely deprecated. NOTE: We do not capture any values, just the name of the option used.

References

• Collect config environment variable usage metric (#571614 - closed) • Michael Eddington • 18.5

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How to set up and validate locally

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Related to #571614 (closed)