Experimental feature: MSC4370 `GET /extremities` endpoint

Tracking issue for https://github.com/matrix-org/matrix-spec-proposals/pull/4370

## Context

T&S is exploring what DAG-level tooling might look like through [policy servers](https://matrix.org/blog/2025/12/policyserv/). This new federation endpoint is expected to be used to allow lightweight server implementations (like [policyserv](https://github.com/matrix-org/policyserv)) to send events and retrieve "current" state information about rooms.

It is *not* enabled by default.

## Current design in Synapse

The endpoint is implemented as a "normal" federation endpoint. Its implementation is copy/pasted from the `make_[join|leave|knock]` endpoints as they return similar information.

## Caveats

The naming might change as the proposal works through the spec process. As of writing, the MSC has only had high level review.

## When to remove

The *unstable* feature can be removed in these cases:

* Immediately upon MSC rejection.
* At least 2 months after a stable implementation (implying MSC acceptance). Can probably be earlier after talking to/convincing T&S.
* Immediately after T&S says it's fine to remove (T&S may need prompting to make this decision)

Start by reading MSC4370 and its linked proposal pull request, then inspect Synapse's existing federation make_join, make_leave, and make_knock endpoint implementations referenced in the issue. The issue does not name files or tests; completion depends on the proposal's status and coordination with T&S about when the unstable feature may be removed.

Written by the indexing model from the issue text.