Add Checker::test_on_with to pass opaque data to lints

[danielbodorin]

Closes #683.

What

Adds a way for a program that embeds selene-lib to pass arbitrary, structured, per-check data to a lint without encoding it in the source being linted.

• AstContext gains an optional opaque (Box<dyn Any>) value plus a typed accessor caller_data::<T>().
• Checker::test_on_with(&ast, Box<dyn Any>) runs the checker with that value attached; test_on is unchanged and equivalent to supplying no caller data.

let diagnostics = checker.test_on_with(&ast, Box::new(MyData { /* ... */ }));

// in a lint's pass():
if let Some(data) = ast_context.caller_data::<MyData>() {
    // use the caller-provided data
}

Why

Lints receive ast, a Context (per-checker, e.g. the standard library) and an AstContext (per-file, e.g. scopes), but there is no channel for the caller to hand a lint its own per-file information (metadata the embedding program knows that isn't in the source). This adds a typed, opaque one.

How

• The value is type-erased via Any and stored on AstContext; only the caller and the lint agree on its concrete type. caller_data::<T>() returns None when no data was supplied or the type does not match.
• test_on and test_on_with share a private run_lints helper, so lint execution is identical either way.
• No change to the Lint::pass signature, so existing lints are unaffected.

Tests and docs

• Unit test covering the absent, present, and mismatched-type cases.
• New "Receiving data from the caller" section in docs/src/contributing.md; CHANGELOG updated.
• cargo test, cargo fmt --check, and cargo clippy pass; existing behavior is unchanged.