Support per-crate `clippy.toml` via `lint_config`

[AlokSwaminathan]

Note: I'm willing to write the PR for this feature, just posting an issue to get maintainer feedback first

Problem

The clippy aspect currently uses a single global clippy.toml for the entire workspace. There is no way for individual crates to provide their own clippy configuration file.

This matters for lints like disallowed-types, too-many-arguments-threshold, type-complexity-threshold, etc. that require clippy.toml for their configuration. A workspace with many crates often needs different thresholds or disallowed-type lists per crate, but today the only option is a single workspace-wide config.

Root cause

The clippy aspect hardcodes CLIPPY_CONF_DIR from a single private attribute:

rust/private/clippy.bzl L281-285:

"_config": attr.label(
    doc = "The `clippy.toml` file used for configuration",
    allow_single_file = True,
    default = Label("//rust/settings:clippy.toml"),
),

Proposed solution

Extend the existing lint_config / LintsInfo mechanism to support a per-target clippy config file. This is the natural place since lint_config already provides per-target clippy flags.