Support MSC3916 by adding unstable media endpoints to _matrix/client (

element-hq#17213)

[MSC3916](https://github.com/matrix-org/matrix-spec-proposals/blob/rav/authentication-for-media/proposals/3916-authentication-for-media.md)
adds new media endpoints under `_matrix/client`. This PR adds the
`/preview_url`, `/config`, and `/thumbnail` endpoints. `/download` will
be added in a follow-up PR once the work for the federation `/download`
endpoint is complete (see
element-hq#17172).

Should be reviewable commit-by-commit.

changelog.d/17213.feature

@@ -0,0 +1 @@
+Support MSC3916 by adding unstable media endpoints to `_matrix/client` (#17213).

synapse/config/experimental.py

@@ -439,3 +439,7 @@ def read_config(self, config: JsonDict, **kwargs: Any) -> None:
         self.msc4115_membership_on_events = experimental.get(
             "msc4115_membership_on_events", False
         )
+
+        self.msc3916_authenticated_media_enabled = experimental.get(
+            "msc3916_authenticated_media_enabled", False
+        )