Tor / proxy support with authentication

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

Currently Elements don't support using a proxy or Tor outside of Element Web flag without authentication.

**Describe the solution you'd like**

I would like Elements to support configuring a proxy (or in case of Element Android supporting connecting to OrBot directly) without obscure command line flags that don't support proxy authentication.

**Additional context**

*I wrote this section in a text editor before seeing the issue template*

I think Element should support avoiding censorship by proxies, especially Tor which by default uses socks5, TCP port 9050 or with Tor Browser 9150.

* Riot-meta: https://github.com/vector-im/riot-meta/issues/287
* [ ] Android: https://github.com/vector-im/element-android/issues/488
* [ ] iOS: https://github.com/vector-im/element-ios/issues/1085
* [ ] Web: https://github.com/vector-im/element-desktop/issues/772 (supported by `--proxy-server=socks5://127.0.0.1:9050` which isn't particularly user friendly and no support for authentication)

Currently proxy authentication is not supported even by Element Web, but I consider that an important feature due to Tor's option `IsolateSOCKSAuth` which allows having different circuits for when different username/password combination is given.

> IsolateSOCKSAuth
> Don’t share circuits with streams for which different SOCKS authentication was provided. (For HTTPTunnelPort connections, this option looks at the Proxy-Authorization and
> X-Tor-Stream-Isolation headers. On by default; you can disable it with NoIsolateSOCKSAuth.)

* https://2019.www.torproject.org/docs/tor-manual.html.en

Review the linked Riot Meta, Element Android, Element iOS, and Element Desktop issues first, then compare the existing Element Web --proxy-server behavior with the requested SOCKS5 authentication. Done means the affected clients have an agreed, user-facing proxy or Tor configuration that supports authentication, with the platform scope resolved.

Written by the indexing model from the issue text.