AthenaKernel+StoreGate: Change storeName to std::string_view

Change the `StoreID::storeName` helper to return a `std::string_view`
and adjust all handles to accept a `std::string_view` instead of `const
std::string&`. This allows the helper function to become `constexpr`. A
few explicit `std::string` conversions had to be added for calls that do
not support `std::string_view`, i.e. construction of ServiceHandles.