Allow filtering available extensions by supporter or support plan

XCOMMONS-3051 added the concept of extension supporter and support plan. The filtering is still technically based on whether the extension is recommended, and what changed is that the extension's recommendation flag is set to true if the extension has at least one support plan attached.

It would be interesting to allow filtering extensions supported by a specific entity or having a specific support plan attached.

See https://forum.xwiki.org/t/improve-information-about-the-level-of-support-of-extensions/13850/40 for a related design discussion.