Embedded / Support sliding sync /syncv3

*This issue was originally created by [**@genofire**](https://github.com/genofire) at .*

**Description:**

We are able to configure "well-know" yet, since #3039 .
But it would be nice, that dendrite support sliding out of the box.
So it would be nice, that the library of sliding-sync is used in dendrite (embedded) or reimplement that API.

Start by reviewing the well-known configuration introduced in #3039 and the current embedded Dendrite setup, then compare the sliding-sync library and API requirements with the project. Done means Dendrite supports sliding sync out of the box in embedded deployments, with the chosen integration or reimplementation covered by appropriate tests.

Written by the indexing model from the issue text.