Support more converter forms in the attrs plugin

[filbranden]

The attrs plugin only recognized converters that were named functions, types, or lambdas. Extend it to also accept:

• Calls returning a callable, e.g. converter=make_converter(arg) — the long-standing request in Unsupported converter, only named functions, types and lambdas are currently supported with higher-order function #15736.
• Variables annotated with a callable type.
• The attrs.Converter wrapper class, including the takes_self and takes_field keyword forms.
• attrs.converters.pipe(...) — the existing Overloaded handling was too strict to recognize its return type.

The init parameter type is still derived from the first positional parameter of the wrapped callable, so existing diagnostics for converters with the wrong signature continue to apply.

Test stubs grew a non-generic Converter class plus pipe / to_bool so the new behavior can be exercised. The field's converter= parameter now accepts Callable | Converter via a new _FieldConverterType alias, which slightly changes the diagnostic message in testAttrsUsingBadConverter[Reprocess].

Fixes #15736.

This was authored with the help of Claude Code.

NOTE: Per test-data/unit/README.md, lib-stub expansion should be avoided and is the kind of change reviewers may want to discuss. Here I have only added the Converter class, _FieldConverterType alias and pipe and to_bool converter functions. These are necessary to test the new converter forms and, as such, I imagine they might be acceptable. Let me know if otherwise.

Checklist:

• Read the Contributing Guidelines
• Add tests for all changed behaviour.
• N/A: If you can't add a test, please explain why and how you verified your changes work.
• Make sure CI passes.
• Please do not force push to the PR once it has been reviewed.

[github-actions]

Diff from mypy_primer, showing the effect of this PR on open source code:

core (https://github.com/home-assistant/core)
+ homeassistant/helpers/entity_registry.py:223: error: Unused "type: ignore" comment  [unused-ignore]

[filbranden]

Diff from mypy_primer, showing the effect of this PR on open source code:

core (https://github.com/home-assistant/core)
+ homeassistant/helpers/entity_registry.py:223: error: Unused "type: ignore" comment  [unused-ignore]

Nice! This is exactly the issue to fix. If this PR goes in and a new release of mypy includes it, I'm glad to follow up with a PR to that project to drop the no longer needed "type: ignore", together with a bump of the mypy version in use there.