Feature request: JSON config format

# Use case

Maddy's uses a custom config format. This makes it hard to integrate maddy with solutions for automatic configuration management (both generating maddy config or parsing maddy config). NixOS for example can very easily generate JSON from it's structured module system, but it is much harder to generate the current maddy config format in a meaningful way from nix.

# Your idea for a solution

I propose to introduce a canonical JSON representation of the configuration and a way to convert from the current configuration format to the JSON format (similar to how Caddy uses the human readable Caddyfile format and a machine readable JSON format). This also makes it easier to write various adapters that can translate from other mailserver config (e.g. dovecot) to the equivalent maddy config on a best effort basis (similar to how Caddy has adapters e.g. for nginx).

- [ ] I'm willing to help with the implementation

该 issue 没有指明文件、测试或入口点。首先了解现有的配置格式，并确定规范 JSON 表示形式以及两个转换方向的范围；完成的标准是模式和转换行为都已定义并实现。

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