Building the `SyncService` can fail if a `/versions` request fails

[poljar]

Since #5618 building a SyncService might fail to build because a /versions requests fails.

The builder attempts to check if support for MSC4306 is enabled on the homeserver.

matrix-rust-sdk/crates/matrix-sdk-ui/src/room_list_service/mod.rs

Lines 160 to 180 in f9584f5

	if client.enabled_thread_subscriptions() {
	let server_features = client
	.supported_versions()
	.await
	.map_err(|err| Error::SlidingSync(err.into()))?
	.features;
	if !server_features.contains(&FeatureFlag::from("org.matrix.msc4306")) {
	warn!(
	"Thread subscriptions extension is requested on the client, but the server doesn't advertise support for it: not enabling."
	);
	} else {
	debug!("Enabling the thread subscriptions extension");
	builder = builder.with_thread_subscriptions_extension(
	assign!(http::request::ThreadSubscriptions::default(), {
	enabled: Some(true),
	limit: Some(ruma::uint!(10))
	}),
	);
	}
	}

Critically, /versions nowadays can also fail due to token expiration, for more info see #5816 and #5822. This combination means that building a SyncService can simply fail due to a network request failing. Just enabling token refresh on the /versions request results in a deadlock: #5825.

Two things that would be nice to achieve:

1. Remove the possibility of a network failure to fail the SyncService builder.
2. Allow token refres for the /versions request.