Is your feature request related to a problem? Please describe.
Support more simpler/faster renaming of keys

Describe the solution you'd like
Support rename_keys processors to rename every key to be lowercase/uppercase. Add support for faster replacements (like replacing "-" with "_") without regex.

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.