Convert Rust node generation to a library

[philpax]

At present, rucomfyui_generate_nodes is a standalone application that will generate node definitions for the standard ComfyUI nodes. However, users (i.e. me) often have custom nodes that they would still like to use in a typed form.

We should refactor the code so that it will take arbitrary ObjectInfos and generate Rust code from them, and then allow use in a build script / macro so that you can easily paste in a definition and generate the code for use in your project.

The unanswered question is: how do we deal with custom types? We could have the macro define these, but I fear that there will be conflicting types. My current thinking is that we just use a separate macro to force the user to use their own common types. That should also allow them to define their own conversions if required.