Support for externally managed redis server

Issue description

What I expected to happen:

Setting mailserver.redis.* to a redis server, which is managed by a different part of my nixos config would not configure services.redis.servers.rspamd as a local redis server.

What happened:

It implies services.redis.servers.rspamd.enable = true and adds a dependency to redis-rspamd.service on rspamd.service.

SNM release branch name and Commit ID

Implemented here: https://gitlab.com/simple-nixos-mailserver/nixos-mailserver/-/blob/master/mail-server/rspamd.nix#L101

Tested with nixos-21.11

Relevant part of the config to reproduce:

mailserver.redis.address = "1.2.3.4";