[CK_BUILDER] Add experimental builder directory and configuration for composable_kernel (#3043)

Add experimental builder infrastructure for composable_kernel

- Add experimental/builder directory with README documentation.
- Create initial test infrastructure with CMakeLists.txt and placeholder test.
- Update root CMakeLists.txt to support CK_EXPERIMENTAL_BUILDER option.
- Update .gitignore to not treat `experimental/builder` as a CMake build directory.

This establishes the directory structure  for a high-level builder pattern that will provide a semantically-clear interface for constructing CK operations, with initial focus on convolution kernels for MIOpen integration.

experimental/builder/test/CMakeLists.txt

@@ -0,0 +1,20 @@
+
+include(gtest)
+
+# Helper function to create a gtest executable with common properties
+function(add_ck_builder_test test_name)
+    add_executable(${test_name} ${ARGN})
+    target_compile_features(${test_name} PRIVATE cxx_std_20)
+    target_include_directories(${test_name} PRIVATE
+        "${PROJECT_SOURCE_DIR}/experimental/builder/include"
+        "${PROJECT_SOURCE_DIR}/include"
+    )
+    target_compile_options(${test_name} PRIVATE
+        -Wno-global-constructors
+        -Wno-c++20-compat
+    )
+    target_link_libraries(${test_name} PRIVATE GTest::gtest_main GTest::gmock)
+endfunction()
+
+add_ck_builder_test(test_conv_builder
+    test_conv_builder.cpp)

experimental/builder/test/test_conv_builder.cpp

@@ -0,0 +1,11 @@
+#include <gtest/gtest.h>
+
+class ConvBuilderTest : public ::testing::Test
+{
+};
+
+TEST_F(ConvBuilderTest, PlaceholderTest)
+{
+    // TODO: Implement actual test
+    EXPECT_TRUE(true);
+}