Fixes https://github.com/element-hq/synapse/issues/18659
This changes the Tokio runtime to be attached to the Twisted reactor.
This way, the Tokio runtime starts when the Twisted reactor starts, and
*not* when the module gets loaded.
This is important as starting the runtime on module load meant that it
broke when Synapse was started with `daemonize`/`synctl`, as forks only
retain the calling threads, breaking the Tokio runtime.
This also changes so that the HttpClient gets the Twisted reactor
explicitly as parameter instead of loading it from
`twisted.internet.reactor`
We do this by shoving it into Rust. We believe our python http client is
a bit slow.
Also bumps minimum rust version to 1.81.0, released last September (over
six months ago)
To allow for async Rust, includes some adapters between Tokio in Rust
and the Twisted reactor in Python.
