Add admin API to get some information about federation status (#11407)

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+# Federation API
+
+This API allows a server administrator to manage Synapse's federation with other homeservers.
+
+Note: This API is new, experimental and "subject to change".
+
+## List of destinations
+
+This API gets the current destination retry timing info for all remote servers.
+
+The list contains all the servers with which the server federates,
+regardless of whether an error occurred or not.
+If an error occurs, it may take up to 20 minutes for the error to be displayed here,
+as a complete retry must have failed.
+
+The API is:
+
+A standard request with no filtering:
+
+```
+GET /_synapse/admin/v1/federation/destinations
+```
+
+A response body like the following is returned:
+
+```json
+{
+   "destinations":[
+      {
+         "destination": "matrix.org",
+         "retry_last_ts": 1557332397936,
+         "retry_interval": 3000000,
+         "failure_ts": 1557329397936,
+         "last_successful_stream_ordering": null
+      }
+   ],
+   "total": 1
+}
+```
+
+To paginate, check for `next_token` and if present, call the endpoint again
+with `from` set to the value of `next_token`. This will return a new page.
+
+If the endpoint does not return a `next_token` then there are no more destinations
+to paginate through.
+
+**Parameters**
+
+The following query parameters are available:
+
+- `from` - Offset in the returned list. Defaults to `0`.
+- `limit` - Maximum amount of destinations to return. Defaults to `100`.
+- `order_by` - The method in which to sort the returned list of destinations.
+  Valid values are:
+  - `destination` - Destinations are ordered alphabetically by remote server name.
+    This is the default.
+  - `retry_last_ts` - Destinations are ordered by time of last retry attempt in ms.
+  - `retry_interval` - Destinations are ordered by how long until next retry in ms.
+  - `failure_ts` - Destinations are ordered by when the server started failing in ms.
+  - `last_successful_stream_ordering` - Destinations are ordered by the stream ordering
+    of the most recent successfully-sent PDU.
+- `dir` - Direction of room order. Either `f` for forwards or `b` for backwards. Setting
+  this value to `b` will reverse the above sort order. Defaults to `f`.
+
+*Caution:* The database only has an index on the column `destination`.
+This means that if a different sort order is used,
+this can cause a large load on the database, especially for large environments.
+
+**Response**
+
+The following fields are returned in the JSON response body:
+
+- `destinations` - An array of objects, each containing information about a destination.
+  Destination objects contain the following fields:
+  - `destination` - string - Name of the remote server to federate.
+  - `retry_last_ts` - integer - The last time Synapse tried and failed to reach the
+    remote server, in ms. This is `0` if the last attempt to communicate with the
+    remote server was successful.
+  - `retry_interval` - integer - How long since the last time Synapse tried to reach
+    the remote server before trying again, in ms. This is `0` if no further retrying occuring.
+  - `failure_ts` - nullable integer - The first time Synapse tried and failed to reach the
+    remote server, in ms. This is `null` if communication with the remote server has never failed.
+  - `last_successful_stream_ordering` - nullable integer - The stream ordering of the most
+    recent successfully-sent [PDU](understanding_synapse_through_grafana_graphs.md#federation)
+    to this destination, or `null` if this information has not been tracked yet.
+- `next_token`: string representing a positive integer - Indication for pagination. See above.
+- `total` - integer - Total number of destinations.
+
+# Destination Details API
+
+This API gets the retry timing info for a specific remote server.
+
+The API is:
+
+```
+GET /_synapse/admin/v1/federation/destinations/<destination>
+```
+
+A response body like the following is returned:
+
+```json
+{
+   "destination": "matrix.org",
+   "retry_last_ts": 1557332397936,
+   "retry_interval": 3000000,
+   "failure_ts": 1557329397936,
+   "last_successful_stream_ordering": null
+}
+```
+
+**Response**
+
+The response fields are the same like in the `destinations` array in
+[List of destinations](#list-of-destinations) response.