Add new plugin for GPU selection via command line

[SaschaWillems]

Description

This PR adds a new plugin that adds GPU selection via command line. With this plugin, a user can now override the default GPU selection and explicitly request a GPU by a given index. This is esp. useful if you want to debug/test samples on GPUs other than your main GPU. I currently use this to test and fix things using swiftshader as a second ICD on my system. This PR also removes the get_first_gpu instance function that wasn't used anywhere.

The plugin itself is pretty simple, and GPU selection is done as follows:

sample instancing --gpu 1

General Checklist:

Please ensure the following points are checked:

• [x] My code follows the coding style
• [x] I have reviewed file licenses
• [x] I have commented any added functions (in line with Doxygen)
• [x] I have commented any code that could be hard to understand
• [x] My changes do not add any new compiler warnings
• [x] My changes do not add any new validation layer errors or warnings
• [x] I have used existing framework/helper functions where possible
• [x] My changes do not add any regressions
• [x] I have tested every sample to ensure everything runs correctly

• [x] This PR describes the scope and expected impact of the changes I am making

  Note: The Samples CI runs a number of checks including:

  • [x] I have updated the header Copyright to reflect the current year (CI build will fail if Copyright is out of date)
  • [x] My changes build on Windows, Linux, macOS and Android. Otherwise I have documented any exceptions