Key backup: Return {hash, count} for key upload requests

This is this tuple that allows the client to check if it has locally all keys of the backup

proposals/1219-storing-megolm-keys-serverside.md

@@ -248,6 +248,7 @@ On success, returns a JSON object with keys:
 
 - `hash` (string): Required. The new hash value representing stored keys. See
 `GET /room_keys/version/{version}` for more details.
+- `count` (number): Required. The new count of keys stored in the backup.
 
 Error codes:
 
@@ -275,7 +276,8 @@ Result:
 
 ```javascript
 {
-  "hash": "abcdefghi"
+  "hash": "abcdefghi",
+  "count": 10
 }
 ```
 
@@ -319,7 +321,8 @@ Result:
 
 ```javascript
 {
-  "hash": "abcdefghi"
+  "hash": "abcdefghi",
+  "count": 10
 }
 ```
 
@@ -367,7 +370,8 @@ Result:
 
 ```javascript
 {
-  "hash": "abcdefghi"
+  "hash": "abcdefghi",
+  "count": 10
 }
 ```