[Vulkan] Enable including GLSL files from custom locations in gen_vulkan_spv (#91913)
@bypass-github-export-checks
To include custom locations when building with buck, use a ```-c gen_vulkan_spv.additional_glsl_paths="..."``` flag where ... is a list of filegroups and source directory paths separated by spaces,
ex. to include the sources added in D41413913, you would use
```
buck build ... -c gen_vulkan_spv.additional_glsl_paths="//xplat/caffe2:test_glsl_src_path_a test_src/a //xplat/caffe2:test_glsl_src_path_b test_src/b"
```
(as shown in the test plan)
Differential Revision: [D41413914](https://our.internmc.facebook.com/intern/diff/D41413914/)
**NOTE FOR REVIEWERS**: This PR has internal Meta-specific changes or comments, please review them on [Phabricator](https://our.internmc.facebook.com/intern/diff/D41413914/)!
Pull Request resolved: https://github.com/pytorch/pytorch/pull/91913
Approved by: https://github.com/mcr229
