rust: expose compilation helpers for custom doctest rules

[jrandolf]

Expose the existing rustdoc action builder, dependency conversion helper and LintsInfo provider through public load paths so custom doctest rules can reuse compiler setup without loading private modules.

The public contract documents required rule attributes, toolchains, the C++ fragment, every returned action field, mutation of caller-supplied Args, and the distinction between build-time and runfiles-based invocations. The action builder now creates a fresh Args object when flags are omitted, so optional lint handling works with its default argument. Provider identities are unchanged.

Build-time invocations pass rustdoc as a File so Bazel can rewrite its executable path when path mapping is enabled. A custom rule using only public loads compiles and executes a native doctest with a normal dependency, a procedural macro and lint configuration. It checks dependency-provider conversion and the identity of the exported lint provider. This is a low-level API proposal; the caller still declares outputs and supplies any cross-platform test runner.

Fixes #4301.

Validation: bazel test //test/unit/public_rustdoc:public_rustdoc_test //test/unit/lint_flags:all (4 tests passed), also with --experimental_output_paths=strip --config=clippy --config=rustfmt (4 tests passed). The path-mapping case failed before the fix. Formatting and whitespace checks passed.