Backends: vulkan: add possibility for custom frag and vertex shaders

[johan0A]

This is my fix I use for: #8271

[ocornut]

I am tempted to accept it (maybe with VkShaderModule inputs instead? so if vkCreateShaderModule() ever adds flags that'd be in control of the user), but it seems like it would be a better general-purpose feature to make it easier to change shaders in the render loop.

[johan0A]

Jus to clarify I dont think this really solves #8271 but it does offer a good workaround for it.

(maybe with VkShaderModule inputs instead? so if vkCreateShaderModule() ever adds flags that'd be in control of the user)

Or with a VkShaderModuleCreateInfo?