Implement new generic reporting API (MSC4457)

https://github.com/matrix-org/matrix-spec-proposals/pull/4457

Description is title :)

* [ ] Experimental feature flag (default disabled)
* [ ] Admin APIs to retrieve those reports (for Coop) - this may be its own set of PRs
* [ ] Tests

Per MSC, the unstable->stable->merged transition is managed as "use unstable until merged". Clients can also hit the endpoint and rely on `M_UNRECOGNIZED` error codes.

(note to maintainers: T&S intends to contribute this, but may need help getting it over the line)