Implement MSC3983 to proxy /keys/claim queries to appservices. (#15314)

Experimental support for MSC3983 is behind a configuration flag.
If enabled, for users which are exclusively owned by an application
service then the appservice will be queried for one-time keys *if*
there are none uploaded to Synapse.

tests/appservice/test_api.py

@@ -105,3 +105,62 @@ class ApplicationServiceApiTestCase(unittest.HomeserverTestCase):
         )
         self.assertEqual(self.request_url, URL_LOCATION)
         self.assertEqual(result, SUCCESS_RESULT_LOCATION)
+
+    def test_claim_keys(self) -> None:
+        """
+        Tests that the /keys/claim response is properly parsed for missing
+        keys.
+        """
+
+        RESPONSE: JsonDict = {
+            "@alice:example.org": {
+                "DEVICE_1": {
+                    "signed_curve25519:AAAAHg": {
+                        # We don't really care about the content of the keys,
+                        # they get passed back transparently.
+                    },
+                    "signed_curve25519:BBBBHg": {},
+                },
+                "DEVICE_2": {"signed_curve25519:CCCCHg": {}},
+            },
+        }
+
+        async def post_json_get_json(
+            uri: str,
+            post_json: Any,
+            headers: Mapping[Union[str, bytes], Sequence[Union[str, bytes]]],
+        ) -> JsonDict:
+            # Ensure the access token is passed as both a header and query arg.
+            if not headers.get("Authorization"):
+                raise RuntimeError("Access token not provided")
+
+            self.assertEqual(headers.get("Authorization"), [f"Bearer {TOKEN}"])
+            return RESPONSE
+
+        # We assign to a method, which mypy doesn't like.
+        self.api.post_json_get_json = Mock(side_effect=post_json_get_json)  # type: ignore[assignment]
+
+        MISSING_KEYS = [
+            # Known user, known device, missing algorithm.
+            ("@alice:example.org", "DEVICE_1", "signed_curve25519:DDDDHg"),
+            # Known user, missing device.
+            ("@alice:example.org", "DEVICE_3", "signed_curve25519:EEEEHg"),
+            # Unknown user.
+            ("@bob:example.org", "DEVICE_4", "signed_curve25519:FFFFHg"),
+        ]
+
+        claimed_keys, missing = self.get_success(
+            self.api.claim_client_keys(
+                self.service,
+                [
+                    # Found devices
+                    ("@alice:example.org", "DEVICE_1", "signed_curve25519:AAAAHg"),
+                    ("@alice:example.org", "DEVICE_1", "signed_curve25519:BBBBHg"),
+                    ("@alice:example.org", "DEVICE_2", "signed_curve25519:CCCCHg"),
+                ]
+                + MISSING_KEYS,
+            )
+        )
+
+        self.assertEqual(claimed_keys, RESPONSE)
+        self.assertEqual(missing, MISSING_KEYS)