Make OPTIONS method on MSC3916 endpoints available without auth

OPTIONS method is usually sent by browser in preflight requests, most of the time we cannot control preflight request to add auth header. Synapse will return a 204 response directly without authentication for those OPTIONS method. According to firefox's documentation, both 200 and 204 are acceptable so I think there is no need to change handler in dendrite. This closes https://github.com/matrix-org/dendrite/issues/3424 No need to add a test because this is just a fix and I have tested on my Cinny Web client personally. ### Pull Request Checklist <!-- Please read https://matrix-org.github.io/dendrite/development/contributing before submitting your pull request --> * [x] I have added Go unit tests or [Complement integration tests](https://github.com/matrix-org/complement) for this PR _or_ I have justified why this PR doesn't need tests * [x] Pull request includes a [sign off below using a legally identifiable name](https://matrix-org.github.io/dendrite/development/contributing#sign-off) _or_ I have already signed off privately Signed-off-by: `arenekosreal <17194552+arenekosreal@users.noreply.github.com>`