Idea: Provide a way to generate the headers with a specific subset of extensions

The vulkan-hpp headers are very large and a very large fraction of that are extensions that the user might not ever use. It would be nice if one could generate the headers and only include specific extensions in the code. This way the header would be easier to read on the eyes, autocomplete would have less irrelevant suggestions, IDEs would have an easier time doing syntax highlighting and builds would be faster.

One example: For the C API [glad2](https://glad.dav1d.de/) provides a way to generate a subset that only includes specific extensions. No extensions (not even swapchain) is 5700 lines in the header and all extensions is 22000 lines. So a ~75% reduction.

Start by comparing the requested subset-generation behavior with glad2, then trace how Vulkan-Hpp currently generates its headers. Define how users would select extensions and verify that generated headers can exclude unselected extensions while retaining the requested API.

Written by the indexing model from the issue text.