libc/spawn: support get/set priority

[anjiahao1]

Summary

• Add two new POSIX spawn attribute helper APIs: posix_spawnattr_getpriority() and posix_spawnattr_setpriority().
• This enables callers to read/write the priority field in posix_spawnattr_t through a stable interface.
• Scope of change is limited to libs/libc/spawn/ and build system integration (CMakeLists.txt / Make.defs).
• Related NuttX Issue reference if applicable: N/A.
• Related NuttX Apps Issue / Pull Request reference if applicable: N/A.

Impact

• Is new feature added? Is existing feature changed? NO / YES (please describe if yes).
  • YES: Adds two new public libc/spawn APIs (additive change).
• Impact on user (will user need to adapt to change)? NO / YES (please describe if yes).
  • NO.
• Impact on build (will build process change)? NO / YES (please describe if yes).
  • YES: New source files are added to libc/spawn build inputs.
• Impact on hardware (will arch(s) / board(s) / driver(s) change)? NO / YES (please describe if yes).
  • NO.
• Impact on documentation (is update required / provided)? NO / YES (please describe if yes).
  • NO.
• Impact on security (any sort of implications)? NO / YES (please describe if yes).
  • NO.
• Impact on compatibility (backward/forward/interoperability)? NO / YES (please describe if yes).
  • NO: API-only addition.
• Anything else to consider or add?
  • N/A.

Testing

I confirm that changes are verified on local setup and works as intended:

• Build Host(s): N/A (not tested; API-only addition).
• Target(s): N/A.
• Command(s): N/A.

Testing logs before change:

N/A (baseline log not captured)

Testing logs after change:

N/A (not tested)

PR verification Self-Check

• This PR introduces only one functional change.
• I have updated all required description fields above.
• My PR adheres to Contributing Guidelines and Documentation (git commit title and message, coding standard, etc).
• My PR is still work in progress (not ready for review).
• My PR is ready for review and can be safely merged into a codebase.

[acassis]

@anjiahao1 please update the Documentation: https://nuttx.apache.org/docs/latest/reference/user/01_task_control.html#c.posix_spawn

[acassis]

Update Documentation