Allow configuring the observation channel and jgroups from dynamic input

When configuring a cluster, it's generally needed to configure the observation channel and JGroups.

The goal of this issue is to ensure that any clustering use case can be covered without having any difference in the files stored in each instance.

It's very likely that nothing actually need to be implemented in practice (for example, if JGroups can always be configured in a way in which all instances have the same configuration), which would remove the block on XWIKI-22853. In this case the outcome will be to document how to configure JGroups that way.