loader: Add dynamic dispatch for unknown device extension entrypoints

GetInstancePorcAddr() is specified to return trampoline entrypoints for all
Vulkan core and extension entrypoints that are dispatched on an instance object
or a child of that instance object. However, typically, device extensions would
be unknown to the loader (don't want to rev the loader everytime an IHV creates
a new device extension).

This patch allows loader to dynamically discover device extension entrypoints
and configure generic trampoline code for these discovered device extensions.