posix: options: create XSI_REALTIME_THREADS Option Group

[cfriedt]

Is your enhancement proposal related to a problem? Please describe.

According to the specs:

The Realtime Threads Option Group is denoted by the symbolic constant _XOPEN_REALTIME_THREADS.
...
If the symbolic constant _XOPEN_REALTIME_THREADS is defined to have a value other than -1, then the following symbolic constants shall be defined by the implementation to have the value 200809L:

_POSIX_THREAD_PRIO_INHERIT
_POSIX_THREAD_PRIO_PROTECT
_POSIX_THREAD_PRIORITY_SCHEDULING
_POSIX_THREAD_ROBUST_PRIO_INHERIT
_POSIX_THREAD_ROBUST_PRIO_PROTECT

This Option Group seems to be missing from Subprofiling Considerations although it is clearly described in the specification.

Most of these Options are not owned by a proper POSIX or XSI Option Group so we can probably create one in Zephyr to cover them.

Describe the solution you'd like

Introduce a new CONFIG_XSI_REALTIME_THREADS Kconfig, option in Kconfig.xsi_realtime_threads.

The chosen name follows that of other XOPEN Options that are also Option Groups listed in Subprofiling Considerations.

Describe alternatives you've considered
Leave it as-is, select each of the orphan options manually.

Additional context
#79816

More details about what each POSIX Option implies can be found here.
https://man7.org/linux/man-pages/man7/posixoptions.7.html