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/types/types.go

@@ -22,7 +22,9 @@ import (
 	"strings"
 	"sync"
 
+	userapi "github.com/matrix-org/dendrite/userapi/api"
 	"github.com/matrix-org/gomatrixserverlib"
+	"github.com/matrix-org/gomatrixserverlib/spec"
 	"github.com/matrix-org/util"
 	"golang.org/x/crypto/blake2b"
 )
@@ -336,3 +338,36 @@ func (r *RoomInfo) CopyFrom(r2 *RoomInfo) {
 }
 
 var ErrorInvalidRoomInfo = fmt.Errorf("room info is invalid")
+
+// Struct to represent a device or a server name.
+//
+// May be used to designate a caller for functions that can be called
+// by a client (device) or by a server (server name).
+//
+// Exactly 1 of Device() and ServerName() will return a non-nil result.
+type DeviceOrServerName struct {
+	device     *userapi.Device
+	serverName *spec.ServerName
+}
+
+func NewDeviceNotServerName(device userapi.Device) DeviceOrServerName {
+	return DeviceOrServerName{
+		device:     &device,
+		serverName: nil,
+	}
+}
+
+func NewServerNameNotDevice(serverName spec.ServerName) DeviceOrServerName {
+	return DeviceOrServerName{
+		device:     nil,
+		serverName: &serverName,
+	}
+}
+
+func (s *DeviceOrServerName) Device() *userapi.Device {
+	return s.device
+}
+
+func (s *DeviceOrServerName) ServerName() *spec.ServerName {
+	return s.serverName
+}