Allow to use a custom LegacyEventConverter

XWIKI-16052 introduced a LegacyEventConverter which is transforming DefaultEvent to LegacyEvent. However right now, this converter defines its own role, so there is no way to provide a custom converter.

It might be a way to solve XWIKI-14044 to allow using a custom converter so that some values from the map parameters (for example) are stored in param3 to 5 in the LegacyEvent.