Support sliding sync

[ulyssa]

The rust sdk includes support for sliding sync behind the experimental-sliding-sync feature. It might be worth exploring what's needed to use it to help with faster syncs.

[Tmpod]

Now that Matrix 2.0 is pretty much ready and that the Rust SDK is finalizing SSS support, this should be picked up again. I might take a look at it soon :)

[kenrachynski]

The experimental-sliding-sync feature flag was removed in matrix-org/matrix-rust-sdk@61dd560 on January 8 and, as such, is included in the matrix-sdk-0.10.0 release on February 4. Since main is already on this matrix-sdk version, this task might actually be complete.

[VAWVAW]

probably related: #392