Decompose RandenPoolSeedSeq from NonsecureURBGBase.

Adjust how the RandenPoolSeedSeq detects contiguous buffers passed to the generate function. Previously it made incorrect assumptions regarding the contiguous concept, which have been replaced with some type-based tests for a small number of known contiguous random access iterator types, including raw pointers.

PiperOrigin-RevId: 452564114
Change-Id: Idab1df9dd078d8e5c565c7fa7ccb9c0d3d392ad2