Feature request: Only enable user chosen Vulkan subset

[zeno-endemann-kdab]

Currently, volk creates function pointers for all Vulkan API and extensions that are defined in the vulkan header. If instead the user could define (I suppose via macro defines) which core version + extension are being used, it would have the following advantages:

• Only functions that the user plans to use are actually resolved, speeding up initialization
• It prevents the user from accidentally using functions from extensions or core versions that were not declared