Support const-qualified service pointers (again)

The following discussion from !526 (merged) should be addressed:

• @leggett started a discussion: (+1 comment)

  I just discovered that we have a lot of code in ATLAS that does this

  Muon::RpcRDO_Decoder::initialize() {
  ...
    const IRPCcablingServerSvc* RpcCabGet = 0;
    StatusCode sc = service("RPCcablingServerSvc", RpcCabGet);

  using a reinterpret_cast<void*> results in an compile time error:

  /bld2/build/v30r2/build/install/GAUDI/22.0.1/InstallArea/x86_64-centos7-gcc62-dbg/include/GaudiKernel/AlgTool.h:133:57: error: reinterpret_cast from type 'const IRPCcablingServerSvc**' to type 'void**' casts away qualifiers
       return service_i( name, createIf, T::interfaceID(), reinterpret_cast<void**>( &svc ) );
                                                           ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

  I don't think it's appropriate to remove the const from user code, as they're not using any non-const methods of the Service.