Faster joins: Support for calling /federation/v1/state (#12013)

This is an endpoint that we have server-side support for, but no client-side support. It's going to be useful for resyncing partial-stated rooms, so let's introduce it.

synapse/http/matrixfederationclient.py

@@ -958,6 +958,7 @@ class MatrixFederationHttpClient:
         )
         return body
 
+    @overload
     async def get_json(
         self,
         destination: str,
@@ -967,7 +968,38 @@ class MatrixFederationHttpClient:
         timeout: Optional[int] = None,
         ignore_backoff: bool = False,
         try_trailing_slash_on_400: bool = False,
+        parser: Literal[None] = None,
+        max_response_size: Optional[int] = None,
     ) -> Union[JsonDict, list]:
+        ...
+
+    @overload
+    async def get_json(
+        self,
+        destination: str,
+        path: str,
+        args: Optional[QueryArgs] = ...,
+        retry_on_dns_fail: bool = ...,
+        timeout: Optional[int] = ...,
+        ignore_backoff: bool = ...,
+        try_trailing_slash_on_400: bool = ...,
+        parser: ByteParser[T] = ...,
+        max_response_size: Optional[int] = ...,
+    ) -> T:
+        ...
+
+    async def get_json(
+        self,
+        destination: str,
+        path: str,
+        args: Optional[QueryArgs] = None,
+        retry_on_dns_fail: bool = True,
+        timeout: Optional[int] = None,
+        ignore_backoff: bool = False,
+        try_trailing_slash_on_400: bool = False,
+        parser: Optional[ByteParser] = None,
+        max_response_size: Optional[int] = None,
+    ):
         """GETs some json from the given host homeserver and path
 
         Args:
@@ -992,6 +1024,13 @@ class MatrixFederationHttpClient:
             try_trailing_slash_on_400: True if on a 400 M_UNRECOGNIZED
                 response we should try appending a trailing slash to the end of
                 the request. Workaround for #3622 in Synapse <= v0.99.3.
+
+            parser: The parser to use to decode the response. Defaults to
+                parsing as JSON.
+
+            max_response_size: The maximum size to read from the response. If None,
+                uses the default.
+
         Returns:
             Succeeds when we get a 2xx HTTP response. The
             result will be the decoded JSON body.
@@ -1026,8 +1065,17 @@ class MatrixFederationHttpClient:
         else:
             _sec_timeout = self.default_timeout
 
+        if parser is None:
+            parser = JsonParser()
+
         body = await _handle_response(
-            self.reactor, _sec_timeout, request, response, start_ms, parser=JsonParser()
+            self.reactor,
+            _sec_timeout,
+            request,
+            response,
+            start_ms,
+            parser=parser,
+            max_response_size=max_response_size,
         )
 
         return body