Allow DpcppExecutor limited by backend and change the type from string to enum type

[yhmtsai]

Current DpcppExecutor can be limited to certain type by string.
It is better to setup a enum type. (note, it should be able to add additional interface without breaking the current one)

Additionally, GPU can use opencl or level_zero backend, but Ginkgo can not limit the selection by backend.
It is good to have the ability.

We can only limit the selection by ONEAPI_DEVICE_SELECTOR if someone looks for workaround for different selection issue between Ginkgo and the other library.