Add support for MSC3030 `/timestamp_to_event` (#2072)
- `/jumptodate` slash command is being worked on in https://github.com/matrix-org/matrix-react-sdk/pull/7372
- Jump to date headers are being worked on in https://github.com/matrix-org/matrix-react-sdk/pull/7339
Related to https://github.com/vector-im/element-web/issues/7677
Part of MSC3030: https://github.com/matrix-org/matrix-doc/pull/3030
Experimental Synapse implementation added in https://github.com/matrix-org/synapse/pull/9445
