Factor out common definition of Tag type (#1793)

... and remove spurious `additionalProperties: true`

data/api/client-server/definitions/tag.yaml

@@ -0,0 +1,24 @@
+# Copyright 2020-2024 The Matrix.org Foundation C.I.C.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+title: Tag
+type: object
+properties:
+  order:
+    type: number
+    format: float
+    description: |-
+      A number in a range `[0,1]` describing a relative
+      position of the room under the given tag.
+    example: 0.25