Implementation of HTTP 307 response for MSC3886 POST endpoint (#14018)

Co-authored-by: reivilibre <olivier@librepush.net>
Co-authored-by: Andrew Morgan <andrewm@element.io>

tests/server.py

@@ -266,7 +266,12 @@ class FakeSite:
     site_tag = "test"
     access_logger = logging.getLogger("synapse.access.http.fake")
 
-    def __init__(self, resource: IResource, reactor: IReactorTime):
+    def __init__(
+        self,
+        resource: IResource,
+        reactor: IReactorTime,
+        experimental_cors_msc3886: bool = False,
+    ):
         """
 
         Args:
@@ -274,6 +279,7 @@ class FakeSite:
         """
         self._resource = resource
         self.reactor = reactor
+        self.experimental_cors_msc3886 = experimental_cors_msc3886
 
     def getResourceFor(self, request):
         return self._resource