Synapse does not reject an attempt to upload keys for device A from device B

This issue has been migrated from [#11720](https://github.com/matrix-org/synapse/issues/11720).

---

Discovered when playing around writing a stress test.

Synapse seems to treat the contents of `/keys/upload` as an opaque blob. But the spec is pretty explicit that it should have [a certain shape](https://spec.matrix.org/v1.1/client-server-api/#post_matrixclientv3keysupload), and that you must include a device_id which matches "the one when logging in".

As proof, I offer the following debug line
```
* 2022-01-10 18:21:09,180 - synapse.handlers.device - 251 - WARNING - GET-4 - DMR: stream_id=6, devices=[{'device_id': 'MEVKUJOZQV* ', 'keys': {'algorithms': [], 'device_id': 'alice_1', 'keys': {}, 'signatures': {}, 'user_id': '@alice:hs1'}}]
```

which came from

```
async def on_federation_query_user_devices(self, user_id: str) -> JsonDict:
stream_id, devices = await self.store.get_e2e_device_keys_for_federation_query(
user_id
)
logger.warning(f"DMR: {stream_id=}, {devices=}")
```

Start at the /keys/upload client-server API handling and compare its validation with the Matrix specification linked in the issue. Also inspect on_federation_query_user_devices, which is shown as producing the malformed device data; done means mismatched device uploads are rejected and the required request shape is enforced.

Written by the indexing model from the issue text.