Use ijson to parse the response to /send_join, reducing memory usage. (#9958)

Instead of parsing the full response to `/send_join` into Python objects (which can be huge for large rooms) and *then* parsing that into events, we instead use ijson to stream parse the response directly into `EventBase` objects.

synapse/python_dependencies.py

@@ -87,6 +87,7 @@ REQUIREMENTS = [
     # We enforce that we have a `cryptography` version that bundles an `openssl`
     # with the latest security patches.
     "cryptography>=3.4.7",
+    "ijson>=3.0",
 ]
 
 CONDITIONAL_REQUIREMENTS = {