thunderbird: add message filters option

[347Online]

Add option to declare account-specific message filters.

Description

Checklist

• Change is backwards compatible.

• Code formatted with ./format.

• Code tested through nix-shell --pure tests -A run.all
  or nix build --reference-lock-file flake.lock ./tests#test-all using Flakes.
  Neither of these are working for me currently, even against master, happy to re-run if there's something I'm missing

• 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

@d-dervishi @hakan-demirli @ethorsoe @javaes

[347Online]

This would not have been possible without this PR as a reference

That said, I think I've made some improvements giving the option a fleshed out type rather than a freeform attrset.

I also went a different direction on the example/test rule.

[khaneliman]

This would not have been possible without this PR as a reference

That said, I think I've made some improvements giving the option a fleshed out type rather than a freeform attrset.

I would always prefer a submodule with freeform support and use defined known options as a guide, though. Allows us to not restrict user's if we haven't configured something explicitly and reduce maintenance burden.

[347Online]

You make a good point. Would extraConfig be sufficient here?

[347Online]

@khaneliman I went ahead and added a text option to provide a completely custom one, or extraConfig to expand on the existing options.