Implement tool to identify external types used in public APIs (#1172)

* Implement tool to identify external types used in public APIs

* Allow specifying which nightly to use

* Move `visitor::is_allowed_type` into `Config`

* Add doc comments

* Use `tracing_attributes::instrument`

* Rename `RefType` to `ErrorLocation`

* Add ability to output a Markdown table

* Allow customization of crate features in analysis

* Work around rustdoc ICE in `aws-smithy-json`

* Integrate with cargo

* Add a readme

* Fix some comments

* Rename `ContextStack` to `Path`

* Verify rustdoc output format version

* Improve error message