[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/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);
+}