Fixes

Small breaking change: Remove the optional second argument from the agrpc::GrpcContext constructor which would incorrectly turn a completion handler with an associated std::allocator into a completion handler that uses the specified argument for allocations. If control over the allocator is needed then resort to Boost.Asio's standard mechanisms: Associate the allocator with the completion handler or boost::asio::require the allocator from the executor.
Bake the choice of ASIO_GRPC_USE_BOOST_CONTAINER into the installed header files to avoid accidental mixing of libraries compiled with and without it. This behavior is also required by vcpkg.

Chore