Context:
In order to be able to search/filter assignees by their id,name or email or candidateGroups by thei name a search endpoint has to be added for both.

AT:

• a search for assignees endpoint is created
• a search for candidateGroup endpoint is created
• the endpoints are documented at https://confluence.camunda.com/display/CO/Assignee & https://confluence.camunda.com/display/CO/Candidate+groups