Add support for `cfg` field for `disallowed_*` lints

Description

Usecase: use pretty_assertions::assert_eq instead of std::assert_eq in cfg(test)
But allow using std::assert_eq in non-test code

You can hack this using #[cfg_attr(test, allow(clippy::disallowed_macros))], but this will affect other macros defined in disallowed_macros when you might only want to do this for a list of macros

Lints affected:

• disallowed_macros
• disallowed_methods
• disallowed_types

Add support to specify cfg field for disallowed items

disallowed-macros = [
  { cfg = "test", path = "std::assert_ne", reason = "use `pretty_assertions::assert_ne` instead" },
  { cfg = "test", path = "std::assert_eq", reason = "use `pretty_assertions::assert_eq` instead" },
]

Version

rustc 1.86.0 (05f9846f8 2025-03-31)
binary: rustc
commit-hash: 05f9846f893b09a1be1fc8560e33fc3c815cfecb
commit-date: 2025-03-31
host: x86_64-unknown-linux-gnu
release: 1.86.0
LLVM version: 19.1.7

Additional Labels

@rustbot label +C-enhancement

Start by locating the configuration handling, implementations, and tests for the disallowed_macros, disallowed_methods, and disallowed_types lints. Compare their existing disallowed-item entries with the proposed cfg field, then verify that cfg=test restricts each listed item as intended without affecting other configurations.