std::span overloads for PipelineBaseCreateInfo

[Tearnote]

PipelineBaseCreateInfo functions like add_spirv() currently take the source as std::vector. This makes sense for sources loaded at runtime, but requires a needless copy if the sources are embedded in the executable. These functions could have variants that accept a std::span, with the requirement that the underlying buffer will live as long as the vuk context. This is low priority but would reduce API friction a little in this scenario.

This would probably need a change to ShaderSource so that it owns a span and optionally a vector of data.