Backend: make user search query filter configurable

[slint]

The search query is currently "statically" set to check for active:true AND confirmed:true.

This can be an issue for instances that want to configure only a subset of users to be searchable (e.g. exclude "light-weight accounts" or user with specific roles) or even loosen-up the query (e.g. include unconfirmed users).

This change can be introduced via a service config variable, and possibly with the use of the FromConfig utility become configurable via usual Invenio config (e.g. USERS_RESOURCES_DEFAULT_SEARCH_FILTER or similar).