Allow custom user-defined data in `get_spirv_key` for shader permutation generation

[Przemog1]

Description

The nbl::this_example::builtin::build::get_spirv_key function should allow custom, user-defined data.

Description of the related problem

Currently, there is no reasonable way to create shader permutations based on data outside of the limit or features structs. For example, such functionality would be needed in the 28_FFTBloom example: https://github.com/Devsh-Graphics-Programming/Nabla-Examples-and-Tests/blob/e5d5ae2ca9137a6966d00aa039f3e6dae7c23fb9/28_FFTBloom/main.cpp#L183-L207.

[devshgraphicsprogramming]

Seems that custom named permutation structs would be needed, same as deviceCaps is generated by the NSC toolchain.