Adds a type-erased response adapter to the public API #554

Job	Run time
CMake msvc-14.2 Debug C++17	7m 21s
B2 gcc-11	5m 28s
CMake msvc-14.2 Release C++17	6m 34s
B2 msvc-14.2	8m 54s
B2 clang-14	5m 37s
CMake gcc-11 17 Debug	8m 53s
CMake msvc-14.3 Debug C++20	6m 21s
B2 msvc-14.3	7m 16s
CMake gcc-11 20 Release	11m 45s
CMake msvc-14.3 Release C++20	6m 37s
CMake clang-11 17 Debug	6m 24s
CMake clang-11 20 Debug	6m 37s
CMake clang-13 17 Release	9m 38s
CMake clang-13 20 Release	9m 52s
CMake clang-14 17 Debug -stdlib=libc++	8m 15s
CMake clang-14 20 Release -stdlib=libc++	11m 31s
	2h 7m 3s

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