Enable TURN by default in packages

This issue has been migrated from [#8251](https://github.com/matrix-org/synapse/issues/8251).

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Voice and video calls require a TURN server, as STUN is not reliable in practice. There are no public TURN servers that I am aware of, so each homeserver must provide one.

Currently, a TURN server is not installed by default. This means that many homeservers do not have a TURN server running. Those who use these homeservers cannot make voice or video calls if they are behind symmetric NAT or a stateful firewall. Configuring a TURN server requires extra manual effort on the part of the administrator, and the server must be configured carefully to avoid security problems.

To solve this problem, I would like for Synapse to enable TURN by default. Container images would bundle a TURN server, while packages would depend on one. The TURN server will be managed by Synapse, without requiring any manual effort on the part of the administrator. This is similar to how NetworkManager and libvirtd both manage dnsmasq, and how NetworkManager also spawns StrongSwan. For privilege separation reasons, it might be preferable for coturn to be a separate service that Synapse controls. This should be possible on Linux distros that use systemd.