Support more callable converters 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. Includes chained calls and overloaded callees (such as attrs.converters.pipe/default_if_none).
• Variables annotated with a callable 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.

Fixes #15736.

This was authored with the help of Claude Code.

Checklist:

• Read the Contributing Guidelines
• Add tests for all changed behaviour.
• Make sure CI passes.
• Please do not force push to the PR once it has been reviewed.

This is a follow up to #21408, with a narrower scope of only supporting more callable converters and not adding support for the Converter class, which can be done in a later follow up or further revision of #21408 once this PR is merged.

[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]

After a new release of mypy that includes this change, I'm glad to follow up with a PR to home-assistant/core to drop the no longer needed "type: ignore", together with a bump of the mypy version in use there.

[filbranden]

@A5rocks Is this what you had in mind with this comment? I'd appreciate it if you would chime in here and possibly review this PR. Thanks!

[github-actions]

According to mypy_primer, this change doesn't affect type check results on a corpus of open source code. ✅