Adds support for tests and benchmarks via Vcpkg features

Enables building unit tests using GTest and benchmarks using Benchmark,
through Vcpkg features.

Adds `tests` and `benchmark` features to `vcpkg.json`,
and adds corresponding CMake logic to control their build based on
Vcpkg manifest features.

vcpkg.json

@@ -14,7 +14,6 @@
       "name": "vcpkg-cmake-config",
       "host": true
     }
-
   ],
   "features": {
     "avx2": {
@@ -26,6 +25,18 @@
       "dependencies": [
         "imgui"
       ]
+    },
+    "tests": {
+      "description": "Build unit-tests using GTest",
+      "dependencies": [
+        "gtest"
+      ]
+    },
+    "benchmark": {
+      "description": "Build benchmarks",
+      "dependencies": [
+        "benchmark"
+      ]
     }
   }
 }