Add an Admin API to fetch an event by ID (#18963)

Adds an endpoint to allow server admins to fetch an event regardless of
their membership in the originating room.

docs/admin_api/fetch_event.md

@@ -0,0 +1,53 @@
+# Fetch Event API
+
+The fetch event API allows admins to fetch an event regardless of their membership in the room it
+originated in.
+
+To use it, you will need to authenticate by providing an `access_token`
+for a server admin: see [Admin API](../usage/administration/admin_api/).
+
+Request:
+```http
+GET /_synapse/admin/v1/fetch_event/<event_id>
+```
+
+The API returns a JSON body like the following:
+
+Response:
+```json
+{
+    "event": {
+        "auth_events": [
+            "$WhLChbYg6atHuFRP7cUd95naUtc8L0f7fqeizlsUVvc",
+            "$9Wj8dt02lrNEWweeq-KjRABUYKba0K9DL2liRvsAdtQ",
+            "$qJxBFxBt8_ODd9b3pgOL_jXP98S_igc1_kizuPSZFi4"
+        ],
+        "content": {
+            "body": "Hey now",
+            "msgtype": "m.text"
+        },
+        "depth": 6,
+        "event_id": "$hJ_kcXbVMcI82JDrbqfUJIHu61tJD86uIFJ_8hNHi7s",
+        "hashes": {
+            "sha256": "LiNw8DtrRVf55EgAH8R42Wz7WCJUqGsPt2We6qZO5Rg"
+        },
+        "origin_server_ts": 799,
+        "prev_events": [
+            "$cnSUrNMnC3Ywh9_W7EquFxYQjC_sT3BAAVzcUVxZq1g"
+        ],
+        "room_id": "!aIhKToCqgPTBloWMpf:test",
+        "sender": "@user:test",
+        "signatures": {
+            "test": {
+                "ed25519:a_lPym": "7mqSDwK1k7rnw34Dd8Fahu0rhPW7jPmcWPRtRDoEN9Yuv+BCM2+Rfdpv2MjxNKy3AYDEBwUwYEuaKMBaEMiKAQ"
+            }
+        },
+        "type": "m.room.message",
+        "unsigned": {
+            "age_ts": 799
+        }
+    }
+}
+```
+
+