Add per-target clippy_config attribute to Rust rules

[zertosh]

Problem

rules_rust exposes the clippy configuration file only via the global
@rules_rust//rust/settings:clippy.toml build setting. There is no way
to attach a different clippy.toml to a specific target.

In a large workspace, a root clippy.toml is often supplemented by
per-subtree configs that scope disallowed-methods, disallowed-types,
and similar config-only lints to specific subsystems. Under Cargo,
clippy-driver walks parent directories from CARGO_MANIFEST_DIR and
finds the nearest config. Under Bazel only the global file applies, so
those subtree-specific lints never fire.

Solution

Add a clippy_config attribute to the common Rust rule attributes
(rust_library, rust_binary, rust_test, rust_shared_library,
rust_static_library, rust_proc_macro). When set, rust_clippy_aspect
points CLIPPY_CONF_DIR at this file's directory instead of the global
build setting. The existing basename validation and input registration
in rust_clippy_action are reused unchanged.

An alternative was to extend rust_lint_config with a clippy_toml
attribute populating LintsInfo.clippy_lint_files. That keeps the
config on the existing lint_config attribute, but CLIPPY_CONF_DIR
accepts a single directory so the aspect would still pick exactly one
file, and the indirection makes BUILD files less obvious than
clippy_config = ":clippy.toml" on the target.

Testing

• Analysis tests in //test/unit/clippy assert CLIPPY_CONF_DIR
  resolves to the global default when clippy_config is unset, to the
  per-target directory when set, that a rust_test(crate = ...) does
  not inherit the setting from the crate under test, and that it honors
  its own clippy_config.
• //test/clippy:clippy_failure_tester covers a per-target source
  config, a per-target generated config, and precedence of
  clippy_config over the global label flag.

This also fixes the BAD_CLIPPY_TOML label in
clippy_failure_tester.sh, which previously made the existing assertion
pass on a package-resolution error rather than the intended lint
failure.