[Feature] 14341 available assignees

[``* `#14341` Available assignees endpoint for API v3``](https://www.openproject.org/work_packages/14341)

Please consider [the related update of the API v3 documentation](https://github.com/opf/op-api-v3-documentation/pull/9).
Just a suggestion, but I favor moving this into it's own spec to avoid the work packages api spec to become bloated.