Adds a type-erased response adapter to the public API #509

Job	Run time
B2 gcc-11	4m 49s
CMake gcc-11 17 Debug	10m 58s
CMake msvc-14.2 Debug C++17	8m 43s
B2 clang-14	4m 10s
CMake gcc-11 20 Release	13m 8s
B2 msvc-14.2	6m 22s
CMake msvc-14.2 Release C++17	8m 33s
CMake clang-11 17 Debug	7m 27s
B2 msvc-14.3	6m 41s
CMake msvc-14.3 Debug C++20	8m 41s
CMake clang-11 20 Debug	8m 13s
CMake clang-13 17 Release	10m 22s
CMake msvc-14.3 Release C++20	8m 53s
CMake clang-13 20 Release	11m 3s
CMake clang-14 17 Debug -stdlib=libc++	9m 46s
CMake clang-14 20 Release -stdlib=libc++	13m 38s
	2h 21m 27s

Provide feedback