Support sending to_device messages in the rust sdk widget driver. (MSC3819)

The rust sdk widget driver already supports all widget actions required for ElementCall.

It is missing to_device messages however.

## MSC:
https://github.com/matrix-org/matrix-spec-proposals/pull/3819

## WIP PR:
https://github.com/matrix-org/matrix-rust-sdk/pull/4856
