This is something we need for your own usage, and I thought I could share it with the community.
I did it as a patch rather than a plugin, maybe I'll switch to a plugin at a later stage.

The patch adds a filter in the Users page of the Administration section.
This filter contains "All" for any authentication mode, "Internal" for users defined in Redmine only, and lists all defined LDAP authentication sources.

It can be useful when many users are defined, and the admin wants to understand easily where the accounts are coming from (there is no column for that), or filter users from a given source only.