Onion service for matrix-client.matrix.org & matrix.org & dendrite.matrix.org

**Is your feature request related to a problem? Please describe.**

When I attempt to use matrix.org over Tor, I often end up being asked for Cloudflare captcha that neither Element or Nheko is able to provide for the client-server API. With Element desktop I only see error about no connection for no reason, with Nheko the terminal output gets flooded by HTML saying it's Cloudflare captcha.

**Describe the solution you'd like**

I would like matrix.org to provide an onion service, preferably single hop one, that I could `mapaddress` in `torrc` and not require client-side changes.

**Describe alternatives you've considered**

Matrix.org could stop using Cloudflare or stop requiring captchas for `https://matrix-client.matrix.org` in Cloudflare configuration?

**Additional context**

* Screenshot will be added when I catch this issue the next time.
* https://github.com/vector-im/element-meta/issues/200

Start by reviewing the requirements for onion access to matrix-client.matrix.org, matrix.org, and dendrite.matrix.org, including the requested single-hop behavior and torrc mapaddress use. Compare this with the linked Cloudflare-captcha context; done means clients can reach the named services over onion addresses without client-side changes.

Written by the indexing model from the issue text.