Support complex numbers in sycl::reduction

[Pennycook]

The sycl_ext_oneapi_complex_algorithms extension adds support for std::complex to algorithms like reduce and reduce_over_group.

During review of #5394, it became clear that adding support for std::complex to sycl::known_identity would have two benefits:

1. It would simplify the implementation of the group algorithms, since known_identity could be used for all supported types
2. It would enable std::complex to be used by sycl::reduction without the user passing an explicit identity

Opening this issue to gauge interest in the idea, and for discussion with reduction implementers (e.g. @v-klochkov) regarding feasibility of extending known_identity in this way.

[v-klochkov]

I agree and think that sycl::has_known_identity should return true for std::plusstd::complex.
That will automatically enable std::complex plus operation for sycl::reductions without explicitly passed identity value.

[uditagarwal97]

Addressed in #8425.