Support using `RUNPATH` to locate layer DSOs

**What enhancement are you suggesting for the Vulkan Loader? Please describe in detail. **

Currently layer DSOs can be found using `LD_LIBRARY_PATH`, `VK_LAYER_PATH`, or the paths determined by the system environment.

I propose extending this behaviour to also allow searching the `RUNPATH` embedded in the executable, in common with other dynamic dependencies. This makes it easier to deploy applications in sandboxed environments where an executable and accompanying libraries can be deployed but the runtime environment cannot be easily modified.

**Is this specific to a single platform?**
Linux, Macos

Start by tracing the Vulkan Loader's existing layer DSO search behavior for LD_LIBRARY_PATH, VK_LAYER_PATH, and system paths on Linux and macOS. Determine how executable-embedded RUNPATH should participate in that search, then verify that layer discovery works in a sandboxed deployment without modifying the runtime environment.

Written by the indexing model from the issue text.