Introduce a separate build setting for the error format used by Clippy

Today, the error format used by Clippy is governed by `rust/settings:error_format`, the same build setting that controls the error format used by rustc. As a result, the format output by both tools is always the same. This can be an issue if a system wants to capture the output of both the Clippy aspect and the underlying rustc compile but use them for different purposes. For example, it is currently not possible to configure Clippy findings to be output as JSON (for further processing) and rustc diagnostics to be output in human-readable form (for direct display).

I therefore intend to introduce a new build setting, `rust/settings:clippy_error_format` to control the output format of Clippy. The existing setting `rust/settings:error_format` will then only control the output format of rustc.

This is a backwards-incompatible change, so I will put it behind a feature flag called `incompatible_change_clippy_error_format`.