Could we add a new wiredtiger_open configuration option to specify arguments passed to the underlying built-in initialization functions?

For example:

builtin_extension_config={zlib=(compression_level=3)}

That allows each extension to be configured separately. I only realized when looking at this that the config key is being passed through to extensions, rather than just the value of the config key as intended. The fix for that is more involved because it requires some extra entries in the extension API.