meli: support "include" statement

[teto]

so one can mix nix generated and manual configs

Description

This is one thing I missed from my own module. @MunsMan

Checklist

• Change is backwards compatible.

• Code formatted with nix fmt or
  nix-shell -p treefmt nixfmt-rfc-style keep-sorted --run treefmt.

• Code tested through nix-shell --pure tests -A run.all
  or nix build --reference-lock-file flake.lock ./tests#test-all using Flakes.

• Test cases updated/added. See example.

• Commit messages are formatted like

  {component}: {description}
  
  {long description}
  

  See CONTRIBUTING for more information and recent commit messages for examples.

• If this PR adds a new module

  • Added myself as module maintainer. See example.

Maintainer CC

[teto]

due to the recent IFD interdiction (which is cool), my initial attempt using readFile on the generated file wouldn't work.
So instead I included the generated config. Because it's less readable I disabled it when not using includes. This feels a bit hackish but this is the best way I could find to keep the best of both worlds.

[teto]

I will abuse my maintainer hat and merge this. Maybe I should add myself as maintainer of the module, anyway I intend to deal with the fallout

[MunsMan]

Works for me, feel free to add yourself as a maintainer.

I'm currently working on more improvements on the options regarding the setup of the mail backend. I will create a PR if ready.