Allow building using external dependencies

[TheJackiMonster]

I had issues building slang as submodule without fetching its dependencies recursively. So I added options to allow building slang fully with externally included targets and headers.

One example for this here:
https://github.com/TheJackiMonster/vkcv-framework/blob/develop/modules/shader_compiler/config/SLANG.cmake

[csyonghe]

@expipiplus1 can you review this? I remember we have options for selecting where to source the spirv headers etc.
Is it possible to consolidate these options a bit further?

[expipiplus1]

Looks like a very useful improvement! thanks!

[csyonghe]

@expipiplus1 can you please do another round of review, thanks.

[expipiplus1]

@TheJackiMonster I made some cleanups and changes, and merged the latest version of slang, it's in my branch here,
https://github.com/expipiplus1/slang/tree/external

If it works for you, please merge it into your PR here and we'll merge if CI passes

[TheJackiMonster]

Should work for me. I just added an option to disable slang-rhi because it's only used for the render-test target. So from my understanding it should not be required for the rest of the build.

But feel free to correct me if I'm wrong with this.

[expipiplus1]

Ok good, I'll take care of fixing the CI and merging from here.\

Thank you!