posix: sched: Implement get APIs for scheduling parameters

Initial implementation of `sched_getparam()` and `sched_getscheduler()`
POSIX APIs as a part of PSE53 `_POSIX_PRIORITY_SCHEDULING` option group.
Both functions are actually placeholders and just return `ENOSYS`
since Zephyr does not yet support processes or process scheduling.

Signed-off-by: Dmitrii Golovanov <dmitrii.golovanov@intel.com>