ATOM-15483 Adding External Project support to the AtomSampleViewer project (#46)

* Adding the EngineFinder.cmake file which is required to allow the
AtomSampleViewer project to be used in an engine-centric manner

Removed the AtomSampleViewer entry from the
shader_global_build_options.json as that doesn't inject the correct
project path when AtomSampleViewer is used as an external project.
The Shader Compiler Preprocessor injects the project path into the
include hierarchy so there it so it doesn't need to be added in the
shader_global_build_options.json

Updated the test_AtomSampleViewer_main_suite.py script to look for the
expected screenshot folder in the project directory instead of assuming
that there is an "AtomSampleViewer" directory in the Engine Root

* Adding commit to the Atom shader_global_build_options.json file to indicate that the project path is always injected into the front of the include paths

Config/shader_global_build_options.json

@@ -5,6 +5,7 @@
     "ClassData": {
         "PreprocessorOptions" : {
             "predefinedMacros": ["AZSL=17"],
+                // The root of the current project is always the first include path.
                 // These include paths are already part of the automatic include folders list.
                 // By specifying them here, we are boosting the priority of these folders above all the other automatic include folders.
                 // (This is not necessary for the project, but just shown as a usage example.)