Allowlist support for deleting entries

Is your feature request related to a problem? Please describe.
As a user, I would like to filter the keys in my Event with an explicit allowlist, and delete all other keys.

Describe the solution you'd like
An additional parameter in the delete_entries processor

delete_entries:
     delete_all_except: [ “key-one”, "key-two”, "key-three” ]

This would result in the Event only containing the three keys above, and all others will be deleted. We could consider having each element be a regex pattern instead of just a string, but this would impact performance, since we would have to match each key against every pattern until there was a match. For keys that don't match any patterns, they would be matched against every pattern in the list. Given that, I am proposing not support regex yet

Describe alternatives you've considered (Optional)
A clear and concise description of any alternative solutions or features you've considered.

Additional context
Add any other context or screenshots about the feature request here.

The request targets the delete_entries processor, but it does not name implementation files or tests. Locate that processor and its configuration and test coverage first; done means an explicit delete_all_except list preserves only the listed keys and verifies the behavior without regex matching.