Migration role from Community Synapse to other homeserver implementations

[luixxiul]

Is your feature request related to a problem? Please describe.
It would be pretty awesome if there was a migration role from the Community Synapse (CS) to other homeserver implementations such as conduwuit, which has been lately integrated by #3967 and commits based on it.

Describe the solution you'd like

Describe alternatives you've considered

Additional context
I know that it is a difficult (or too risky) task to set up migration. I'm not sure if it would be even possible, but I feel the vendor lock-in to the Community Synapse would not quite be ideal to be honest.

[spantaleev]

Unfortunately, each homeserver implementation has its own very different database format and there are not migration tools that can convert from one to the other.

It's probably up to homeserver developers (who know at least one homeserver implementation's internals well) to create such a tool. Such an undetaking would be doubly-painful for the rest of us.

The fact that various homeserver implementations have appeared over the years and not a single such conversion tool has appeared is probably a nice indicator that it's an extremely difficult task.

The first homeserver implementor to bring both good enough feature-parity with Synapse and a conversion tool will probably win over a ton of users. I suppose everyone working on a homeserver knows this, but.. it's just too difficult to chase both of these (or even one of these!) at the same time.