Context:
In order to enrich userTask assignee/candidateGroup reports and filter values with metadata like First and LastName of the assignee we need to retrieve the metadata of all known assignees and candidateGroups.

AT:

• a dedicated assignee/candidateGroup metadate cache is built up by iterating all distinct known assignee and candidate group ids
  • at Optimize startup
  • as well as in the same intervals as the `UserIdentityCacheService`
• the sync behavior is the same as for the `UserIdentityCacheService`

Out of scope:
a dedicated configuration will be added with OPT-4633