Install obfs4proxy from the Tor Browser tarball

The main goal here is to ship an up-to-date obfs4proxy, since the version in Debian is vastly outdated and has a different network fingerprint than the most commonly used by Tor clients, which is the one shipped in Tor Browser.

In particular, we want to fix the Elligator2 bug (#18800 (closed)). This will be achieved automatically, thanks to this commit, once we upgrade to Tor Browser which has upgraded obfs4proxy: https://gitlab.torproject.org/tpo/applications/tor-browser-build/-/issues/40416

And while I'm at it, I'm installing all Pluggable Transports shipped in the Tor Browser tarball. Currently this means we also install the snowflake-client binary, which is useless right now but should save us some time when we work on #5494. And more generally, this approach should make it slightly easier for us to pick up support for PTs newly added in Tor Browser.

refs #18800 (closed), #5494