Implement MSC 3981 (#3248)
* Implement MSC 3891
* Add necessary mocks to tests
* Only set recurse parameter if supported
* fix: address review comments
* task: unify unstable prefix code between client and tests
* Add test for relations recursion
* Make prettier happier :)
* Revert "task: unify unstable prefix code between client and tests"
This reverts commit f7401e05
* Fix broken tests
