Move raw API and event schemas into /data directory

Historical note: this was originally a series of several commits, spread out
over several weeks. They have been squashed together to make `git annotate`
work properly.

The original commits were:
 * 91ab3934 <Will> 2021-01-25 21:16:42 -0800 Add raw API end event schemas into /data directory
 * aae22f47 <Will> 2021-01-25 21:33:06 -0800 Remove non-data files
 * 1092d4ca <Will> 2021-01-26 20:41:33 -0800 Add data-compatiuble extension (.yaml) to all data files that currently omit one
 * 21060109 <Will> 2021-01-26 20:57:28 -0800 Remove symlink to event-schemas, and update openAPI schema paths accordingly
 * 4f633845 <Travis Ralston> 2021-04-12 21:54:54 -0600 Fix event schema examples too
 * 301c7b2f <Will> 2021-02-05 10:15:42 -0800 Restore docs describing OpenAPI extensions that we use

data/api/server-server/event_auth.yaml

@@ -0,0 +1,74 @@
+# Copyright 2018 New Vector Ltd
+#
+# 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.
+
+swagger: '2.0'
+info:
+  title: "Matrix Federation Event Authorization API"
+  version: "1.0.0"
+host: localhost:8448
+schemes:
+  - https
+basePath: /_matrix/federation/v1
+consumes:
+  - application/json
+produces:
+  - application/json
+securityDefinitions:
+  $ref: definitions/security.yaml
+paths:
+  "/event_auth/{roomId}/{eventId}":
+    get:
+      summary: Get the auth chain for a given event
+      description: |-
+        Retrieves the complete auth chain for a given event.
+      operationId: getEventAuth
+      security:
+        - signedRequest: []
+      parameters:
+        - in: path
+          name: roomId
+          type: string
+          description: The room ID to get the auth chain for.
+          required: true
+          x-example: "!abc123:matrix.org"
+        - in: path
+          name: eventId
+          type: string
+          description: The event ID to get the auth chain of.
+          required: true
+          x-example: "$helloworld:example.org"
+      responses:
+        200:
+          description: The auth chain for the event.
+          schema:
+            type: object
+            properties:
+              auth_chain:
+                type: array
+                description: |-
+                  The full set of authorization events that make up the state of
+                  the room, and their authorization events, recursively. Note that
+                  events have a different format depending on the room version -
+                  check the `room version specification`_ for precise event formats.
+                items:
+                  type: object
+                  title: PDU
+                  description: |-
+                    The `PDUs <#pdus>`_ contained in the auth chain. The event format
+                    varies depending on the room version - check the `room version specification`_
+                    for precise event formats.
+                  properties: []
+                  example:
+                    $ref: "examples/minimal_pdu.json"
+            required: ['auth_chain']