Filtering participants based on their skills

**Is your feature request related to a problem? Please describe.**
Not possible to search participants based on their skills

**Describe the solution you'd like**
add a field that makes it possible to filter based on their skills (this information is already in their profiles)

**Additional context**

No files, tests, or entry points are named. Start by locating the participant profile data and the existing participant search or filtering flow; done should mean users can filter participants by skills already present in their profiles and the results update correctly.

Scritto dal modello di indicizzazione a partire dal testo della issue.