environment variable to save configured settings in a file

Some application vendors configure the validation layer programmatically (i.e. via switches like `-vulkan_enable_validation -vulkan_enable_sync_validation` etc.).

For local analysis, it would be very helpful to know exactly how the validation layer is actually being configured in these cases.

Defining an environment variable `VK_VALIDATION_SAVE_SETTINGS=/path/to/file/to/save/settings` that caused VVL to write a settings file to the given location would make it easy.