Add pattern restriction

Co-authored-by: Andrew Morgan <1342360+anoadragon453@users.noreply.github.com>

data/api/client-server/definitions/m.oauth_params.yaml

@@ -24,6 +24,7 @@ properties:
       A URL pointing to the homeserver's OAuth account management web UI
       where the user can approve the action. MUST be a valid URI with scheme
       `http://` or `https://`, the latter being RECOMMENDED.
+    pattern: "^https?://"
 example: {
     "url": "https://example.org/account/reset-cross-signing"
 }