[NFCI] Move reductions implementation from handler.hpp to reduction.hpp (#7241)

This is done by limiting friend access to `sycl::handler` to two helper
functions:

  - `sycl::detail::reduction::finalizeHandler`
  - `sycl::detail::reduction::withAuxHandler`

so that rest of the code could be moved out of handler.hpp.

Author: aelovikov-intel