collections: make collection class configurable

[alejandromumo]

          Minor: instead of hardcoding the collection class (line 34), we could have added it to the ServiceConfig, as normally done, so it could be more easily overridden.

Originally posted by @ntarocco in inveniosoftware/invenio-rdm-records#1853 (comment)

[github-actions]

This issue was automatically marked as stale.