Wish: override `resolv.conf` with custom resolvers

This issue has been migrated from [#10979](https://github.com/matrix-org/synapse/issues/10979).

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## Preamble

As described in https://matrix.to/#/!XaqDhxuTIlvldquJaV:matrix.org/$m59cU5FR-fl6Rhl8132sdgoqHTZDBmXAEf2KP9FPXAc?via=matrix.org&via=vector.modular.im&via=beeper.com I am currently looking at opennic integration for synapse.

The Idea is simple. Have a config flag in synapse that makes synapse additionally to the system wide servers also check fed on the opennic dns servers.

OpenNic can be found at https://www.opennic.org

Please note I am in no way a direct representative of that project but instead I am simply a user of it and matrix. (Mentioning this to avoid the feeling of having a conflict of interest)

## Why having Opennic in Synapse makes sense while TOR doesnt get integrated

OpenNic is a project providing a community maintained and free root dns zone. This in my opinion fits very much the motive Matrix as a protocol has. It archieves an open, not money based, and federated DNS network where everyone can participate for free on.

While TOR does provide something similiar the Opennic project doesnt require any special setup apart from setting a different DNS server. So the integration usually is as easy as editing `/etc/resolv.conf`.

So you may ask why synapse should integrate this.

The reason I think it is good is as simple: It lowers the barrier needed to make it possible to properly federate with people using opennic domains. Usually you need a jump server for that as other servers will never be able to access the domain without additional setup which would even affect the whole server. By making it an option in synapse it a) would not affect the whole system b) people know it is available but are not forced to support it c) Turning a config flag is done faster as you do not have to figure out which dns server to use.

## Additional notes:

This issue is primarily an open discussion at this time if this makes even sense to have as this likely is controversial. Especially as it may or may not be a bigish diff for the code of synapse to add a fallback dns to the regular resolving on federation.

My personal wish is this would fit the motive of matrix quite well and is much easier to support than TOR.