Add DeviceScan::*ByKey env algorithms (#7970)

* Add DeviceScan::*ByKey env algorithms

* * Remove scan_by_key_impl_env helper, inline dispatch_with_env directly
* Rename InitT to init_t in env overloads
* Use plus<init_t>/plus<value_t> instead of plus<> to avoid integer promotion

* Fix api tests so that they use examples where env is passed epxlicitly with stream

Author: gonidelis