Enable guest access on new media endpoints, per MSC4189

[turt2live]

I didn't see tests for other guest access endpoints in my 15 seconds of searching, but if there's guidance on how to set that up, happy to add the respective tests.

[anoadragon453]

It appears that we don't have any unit tests for guest access. Sytest appears to be where guest access was historically tested. If you're feeling adventurous, you could add a subtest to https://github.com/matrix-org/complement/blob/39733c1b2f8314800776748cc7164f9a34650686/tests/csapi/apidoc_content_test.go#L32-L56, which is where authenticated media appears to be primarily tested.

I don't believe this needs an experimental features option due to the simplicity. The MSC looks poised to go through and disabling this in production is easy given the small code size.

Feel free to add a test in Complement to verify, but for now this lgtm.

[turt2live]

I am feeling adventurous, but unfortunately my bandwidth constraints prevent realizing that interest :(

maybe one day, though.