clippy: add settings.allowedLints

[anstylian]

My project builds with stable cargo 1.96.0, but the clippy hook runs nightly, which knows lints stable doesn't — e.g. clippy::unused_async_trait_impl. With denyWarnings the hook fails on them, and I can't #![allow] the lint in code because stable then complains about an unknown lint.

This adds settings.allowedLints, rendered as -A <lint> after -D warnings so it composes with denyWarnings:

  hooks.clippy = {
    enable = true;
    settings = {
      denyWarnings = true;
      allowedLints = [ "clippy::unused_async_trait_impl" ];
    };
  };

Default is [], so existing configs are unchanged.