Adds a type-erased response adapter to the public API #502

Job	Run time
B2 gcc-11	4m 20s
B2 msvc-14.2	6m 5s
B2 clang-14	3m 41s
CMake msvc-14.2 Debug C++17	6m 38s
B2 msvc-14.3	3m 0s
CMake gcc-11 17 Debug	10m 48s
CMake msvc-14.2 Release C++17	9m 10s
CMake gcc-11 20 Release	13m 42s
CMake msvc-14.3 Debug C++20	3m 1s
CMake clang-11 17 Debug	7m 35s
CMake msvc-14.3 Release C++20	2m 51s
CMake clang-11 20 Debug	7m 39s
CMake clang-13 17 Release	10m 26s
CMake clang-13 20 Release	10m 55s
CMake clang-14 17 Debug -stdlib=libc++	9m 36s
CMake clang-14 20 Release -stdlib=libc++	13m 46s
	2h 3m 13s

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