I don't really like the attribute name; in any case, we should strive to cause least pain for people who are expecting the existing behavior (replacing the converter list). So, when `message-converters` is supplied, this should default to false - that will be difficult to document (because when there's no `message-converters` the default is true.

Maybe `retain-default-converters` ? That way, it's clear(er) that this only applies when `message-converters` is supplied.

Plus, we would need code in the parser to only allow this attribute if `message-converters` is also supplied (remove the default from the schema).

I wonder if it would be better to have `message-converters` (replaces the default) and `additional-message-converters`, `extra-message-converters`, `supplemental-message-converters` or similar (added before the defaults).