[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).
Using the Watchers representer strikes me as weird, we should probably rename this Representer to be a UsersRepresenter.

And then there is the issue of the WatchersRepresenter being different from the representations implemented by @myabc and myself in https://github.com/opf/openproject/pull/1634/files. We should harmonise the representation.