de-MSC-ifying space summaries (MSC2946) (#3134)

- This PR moves and refactors the
[code](https://github.com/matrix-org/dendrite/blob/main/setup/mscs/msc2946/msc2946.go)
for
[MSC2946](https://github.com/matrix-org/matrix-spec-proposals/pull/2946)
('Space Summaries') to integrate it into the rest of the codebase.
- Means space summaries are no longer hidden behind an MSC flag
- Solves #3096

Signed-off-by: Sam Wedgwood <sam@wedgwood.dev>

roomserver/roomserver.go

@@ -28,6 +28,9 @@ import (
 )
 
 // NewInternalAPI returns a concrete implementation of the internal API.
+//
+// Many of the methods provided by this API depend on access to a federation API, and so
+// you may wish to call `SetFederationAPI` on the returned struct to avoid nil-dereference errors.
 func NewInternalAPI(
 	processContext *process.ProcessContext,
 	cfg *config.Dendrite,